Aaron, The First High Priest of Israel
The Book of Exodus informs us that Aaron, the first High Priest, was born to Jochebed - the Egyptian-born daughter of Levi and her nephew Amram. Jochebed and Amran's eldest child was Miriam, followed by Aaron then, about three years later, Moses. The Bible tells us that Aaron married Elisheba, who bore him four sons - Nadab and Abihu, Eleazar and Ithamar. (Exodus 6:23)
Acting in tandem with his brother Moses, Aaron was a crucial part of the freeing of the nation from slavery in Egypt. Although Moses was the actual leader, he was slow of speech and slow of tongue so God chose his fluent brother Aaron to go with Moses to confront the Pharaoh and act as Moses' 'mouth'. It was Aaron who by various signs convinced the elders of Israel that the Lord had seen the affliction of the people and was going to do something about it (Exodus 4:27-31).
After three chapters detailing the construction of the Tabernacles, In Exodus 28:1, the Lord told Moses to bring his brother Aaron and his sons near to him so they could minister as priests to Him. The rest of the chapter is devoted to the garments Moses was instructed to have made for them
The importance of the office of High Priest is exemplified by the fact that the garments were (with the exception of gold) made of the same materials as the the veil of the tabernacle and the curtain that covered the entrance of the tent. See Exodus 25:4, Exodus 26:1,31, 36, etc.
Exodus 29 records the ceremony by which Aaron and his sons were consecrated as priests to the Lord. After the sacrifices were made and the men anointed, Aaron and his sons were commanded to stay at "the doorway of the tent of meeting for seven days" because their ordination would take that amount of time. At the end of the week Moses' brother and nephews became the go-between between God and the people - an unimaginably great responsibility.
Note: this did not mean that no priests existed before this, but that the Aaronic Priesthood was officially instituted. In Exodus 19:22-24, then the nation was camped at the foot of Mt. Sinai the Lord warned
"Also let the priests who come near to the Lord consecrate themselves, or else the Lord will break out against them." Moses said to the Lord , "The people cannot come up to Mount Sinai, for You warned us, saying, 'Set bounds about the mountain and consecrate it.'" Then the Lord said to him, "Go down and come up again, you and Aaron with you; but do not let the priests and the people break through to come up to the Lord , or He will break forth upon them."
However, in spite of his large role in the events leading up to the Exodus, and the fact that he and his sons were already chosen to be priests to the Lord, Aaron is never portrayed as irreproachable. Much to the contrary, during Moses' prolonged absence on Mount Sinai, Aaron yielded to the pressure of the people and used their jewelry to make a golden calf that he told them was the god who had delivered them from Egypt. See Footnote I
Unfortunately, it didn't take long for things to go terribly wrong in this privileged family. While it is was a tragedy of huge proportions it behooves us to understand why Nadab and Abihu - the two oldest sons - were struck dead by God Himself. Also the reason why the two younger brothers, Eleazar and Ithamar (and possibly Aaron as well) knowingly did not comply with one of God's commandments but were not even seen as guilty, much less struck down. Much to the contrary, God blessed their lives and the lives of their descendants.
Two perished, two prospered! This inconsistency has deeply troubled theologians for a very long time.
Eleazar and Ithamar - Aaron's Youngest Sons
The two that prospered were Eleazar and Ithamar through whom the priestly line is traced because neither of the two older sons Nadab and Abihu had any children (Numbers 3:4).
We do not know much about Ithamar the youngest son, except that he was the treasurer of the offerings for the tabernacle (Exodus 38:21). Also when Moses was instructed to put the descendants of Kohath, Gershon and Merari, to do the work connected with the tent of meeting and the tabernacle, Ithamar was put in charge of the sons of the Gershonites and the sons of Merari who collectively numbered about 7,000.. (Numbers 4).
Ithmar was also the father of the priestly line to which Eli, a Jewish priest in the days of the judges belonged. While Eli Himself seemed to be a righteous man, he did not take steps to stop his two sons from desecrating the temple. As a result God told Eli in 1 Samuel 2, that the priesthood would depart from his house. Fast forward to the days of Solomon and the priest Abiathar who was of Eli's family. Although Solomon spared his life, he dismissed Abiathar from being priest (1 Kings 2:26-27), thus (unintentionally?) fulfilling the prophecy made a century earlier.
However, Ithamar's lineage continued through a man named Daniel who was one of the many who returned from the Babylonian captivity (Ezra 8:2)
Eleazar was Aaron's third son. As the oldest surviving son, he was "the chief of the leaders of Levi", who oversaw those who carried out the duties of the sanctuary, (Numbers 3:32). Additionally he was responsible for "...the oil for the light and the fragrant incense and the continual grain offering and the anointing oil--the responsibility of all the tabernacle and of all that is in it, with the sanctuary and its furnishings." (Numbers 4:16 NASB)
Eleazar also became High Priest after Aaron's death. (Numbers 20:23-29; Deuteronomy 10:6), and it was in his presence that Moses commissioned Joshua as the new leader of the nation.
"...he shall stand before Eleazar the priest, who shall inquire for him by the judgment of the Urim before the Lord. At his command they shall go out and at his command they shall come in, both he and the sons of Israel with him, even all the congregation." Moses did just as the Lord commanded him; and he took Joshua and set him before Eleazar the priest and before all the congregation. Then he laid his hands on him and commissioned him, just as the Lord had spoken through Moses. (Numbers 27:18-23 NASB)
After entering the promised land, it was Eleazar, Joshua, and the heads of the households who apportioned the land to the tribes as their inheritance, (Joshua 14:1 NASB)
Eleazar married one of the daughters of Putiel, and had a son called Phinehas (Exodus 6:25), who eventually took over as High Priest (Joshua 24:33)
Phineas had a long and distinguished career, first coming to prominence at Shittim when "the people began to play the harlot with the daughters of Moab". Phinehas killed the Israelite Zimr and Cozri, a Midianite woman whom Zimri had brought into the camp. This event (commemorated in Psalms 106:30 and 31) turned away God's wrath in the form of a plague that was rampaging unabated through the ranks. Phineas' action prompted the Lord to say
'Behold, I give him My covenant of peace; and it shall be for him and his descendants after him, a covenant of a perpetual priesthood, because he was jealous for his God and made atonement for the sons of Israel.'" (Numbers 25:113 NASB)
Phinehas was also chief adviser in the war with the Benjamites. It would however, take a dedicated page to list all his achievements.. It is well worthwhile to look up his name in a good Bible dictionary..
Suffice to say that the scribe Ezra was a descendant of Aaron through Eleazar and Phinehas (Ezra 7:1-5). A year after the completion of the Second Temple by Jews who had returned to the Land of Israel, Ezra led a second wave of exiles back from Babylon.
Ezra's importance cannot be overstated. As "a scribe skilled in the law of Moses" (Ezra 7:6), it fell to him to teach the Lord's "statutes and ordinances in Israel" (Ezra 7:10).
The first wave of returnees, including Zerubbabel and Nehemiah (Ezra 2:1-2), built the altar of God on its original foundation and offered burnt offerings on it to the Lord. They also laid the foundation of the temple itself (Ezra 3). The problem was that they had apparently forgotten (or were ignoring) the Lord's repeated commands not to intermarry with the pagan nations around them and thus fall into unacceptable ways. Because the people were taking spouses from a laundry list of nations, including the Canaanites, the Hittites, the Perizzites, the Jebusites, the Ammonites, the Moabites, the Egyptians and the Amorites (Ezra 9) they were no longer separate from the abominations of these nations.
When Ezra discovered this he was so shocked and appalled that he tore his garments and pulled the hair out of his head and beard and fell on his knees in a long and agonized prayer (Read Ezra 9). Finally the people confessed that they had transgressed greatly and covenanted with God to do as Ezra instructed, i.e. separate themselves from the peoples of the land and their foreign wives - which they did over a period of time (Read Ezra 9-10).
Without a return to all the laws of Moses, the rebuilt temple would have been just another building, and Jerusalem just another walled city.
See What is Holiness
In the 21st century, the word "holy" can convey some very negative connotations - often used to describe someone who is self-righteous, smug, sanctimonious, goody-goody, priggish etc. To most Christians, the word "holy" implies moral goodness. However, this is only part of the meaning. While holiness certainly implies goodness, the core meaning is not good but set apart -- and therefore, good
Danielís prophecy that specified EXACTLY when the Messiah would appear is connected with Ezra's return to Jerusalem. See THIS page (Scroll Down)
Nadab and Abihu - Aaron's Oldest Two Sons
Nadab and Abihu were Aaron's sons, Moses' nephews and direct descendants of Abraham, Isaac and Jacob. Not only were they from 'good stock' as it were, but they seemed to have been in an especially privileged position with benefits and experiences that few in the history of mankind can ever imagine. They were witnesses to all the miracles God used to bring about the Exodus They felt the whole mountain quake and saw thunder, lightning, and smoke ascend as from a furnace when the Lord descended on Mt. Sinai. They literally heard the voice of God.
And that is far from all.
When the nation of Israel was encamped at the foot of Mt. Sinai, the Lord told Moses to
"Come up to the Lord, you and Aaron, Nadab and Abihu and seventy of the elders of Israel, and you shall worship at a distance. (Exodus 24:1 NASB)
Nadab and Abihu were not only singled out by name to go up on the mountain, but they were listed as being among "the nobles of the sons of Israel" who saw the God of Israel and ate and drank in His presence.
Then Moses went up with Aaron, Nadab and Abihu, and seventy of the elders of Israel, and they saw the God of Israel; and under His feet there appeared to be a pavement of sapphire, as clear as the sky itself. Yet He did not stretch out His hand against the nobles of the sons of Israel; and they saw God, and they ate and drank. (Exodus 24:9-11 NASB)
Additionally, one of them most certainly would been High Priest after Aaron..
Note: Many will go to great lengths to explain how 'seeing God' doesn't really mean seeing God. However, in the Old Testament, the list of people who claim that they had 'seen' God is rather extensive, and includes Abraham, Jacob, Moses, the afore mentioned 74 Elders, and the prophets Isaiah, Ezekiel, and Daniel. Even the little known prophet Micaiah gave his seeing the Lord as his credentials for being a true prophet. The problem actually has a very simple solution. See Has Anyone Ever Seen God?
And Their Fall
Unfortunately, neither of these two men lived up to their position or potential. Nadab and Abihu came to a rather a sudden and grisly death at the hands of the Father. Leviticus 9:24 describes how fire from the Lord consumed the burnt offering, but the very next verse describes fire from heaven consuming the two priests. Leviticus 10:1 offers us a very brief account of what happened
Now Nadab and Abihu, the sons of Aaron, took their respective fire pans and, after putting fire in them, placed incense on it and offered strange fire before the Lord, which He had not commanded them. And fire came out from the presence of the Lord and consumed them, and they died before the Lord. (Leviticus 10:1-2 NASB)
It seems reasonable to assume that the "strange fire" the brothers offered was on the altar of incense.
This altar two cubits high made of acacia wood and overlaid with pure gold, stood outside the veil in the holy place (not in the Holy of Holies). As its name suggests the altar was reserved for fragrant incense that Aaron was to burn on this altar twice every day when he trimmed the lamps- thus there was to be perpetual incense before the Lord.
Since both Nadab and Abihu were put to death for the same offense, there obviously had to have been some sort of collusion between them. The swift extermination of two men who were not only priests, but Aaron's son, must have sent shock waves through the entire camp, convincing the Israelis that the priesthood had to be one of the most dangerous jobs on earth.
But what exactly did they do wrong? Leviticus 10 says they "offered strange fire before the Lord". What the "strange fire" was is not entirely clear, but theories abound.
"Law of Silence"
Many churches and individual believers have understood the fate that befell these two men to be a strict warning against any kind of innovation during worship. Just as God did not tolerate the two men doing something that He had not specifically commanded, He would take an equally dim view of us introducing some kind of "strange fire" that He has not specifically commanded..
This is absolute nonsense. Anyone who believes this is not reading everything pertaining to this incident.
God was not even remotely silent on matters relating to the Temple and its rituals. In fact, He was so clear about how, when, where, why and by whom the rituals were to be carried out, that if the priests made any substitutions at all they were actually violating very clearly expressed directions - not any imaginary "Law of Silence".
Yes, Leviticus 10:1-2 says Nadab and Abihu "offered strange fire before the Lord, which He had not commanded them." But in Exodus 30:9, after the Lord specified the design and size of the altar of incense, He very clearly told them that they were not to "offer any strange incense on this altar", nor were they to offer a burnt offering, a meal offering, nor a drink offering on it.
"You shall not offer any strange incense on this altar, or burnt offering or meal offering; and you shall not pour out a drink offering on it. (Exodus 30:9 NASB)
What Nadab and Abihu did was in direct violation of very specific instruction issued by the Lord. Thus the words "which He had not commanded them" (Leviticus 10:1-2)was another way of saying they did something He had explicitly told them not to do.
Yet, in spite of the clear evidence, people persist in clicking their tongues when, according to them, someone violates this so called "Law of Silence". People have come up with a huge variety of practices that supposedly fall under the very broad heading of "strange fire". They range from not keeping the Sabbath, using certain (or all) musical instruments, women pastors, coming late to service (you read that right) etc. etc. etc. As Al Maxey wrote,
If I had a dollar for every time those who promote patternism have resurrected Nadab and Abihu to bear witness to the deadly consequences of "innovations" in worship, I could have retired years ago. "Remember Nadab and Abihu!" has been the mantra of rigid religiosity for generations. Those who oppose eating a meal in the church building will quickly cry out, "Remember Nadab and Abihu!" If the teens clap during a song in the "worship assembly" they will need to be reminded of two sons of Aaron "on fire for the Lord." If we support an orphan out of the "treasury," or use more than one cup in the Lord's Supper, or employ four part harmony in our singing, or any one of a thousand other "insidious innovations," we are quickly warned that we tread the same path as Abihu and his brother Nadab. 
I guess it has never been properly understood that virtually everything connected with our method of 'doing church' including the specially built, often very fancy and extremely expensive brick and mortar buildings. The non-participatory and completely passive 'services' is a modern innovation that would have had 1st century Christians scratching their heads. The 'clergy' as a body of ordained religious practitioners did not exist in the New Testament, neither did priests. In fact, by first century standards, the entire church 'service' would be very strange fire indeed. See The Church ...Then And Now
So let us leave aside the "Law of Silence" hypothesis and take a look at the other three most common theories.
1) Fire is Fire
Nadab and Abihu, thinking that all fire is the same, did not fill their fire pans with coals from the altar of burnt offering (on which the fire was always burning) then add the incense.
However, the only time filling one's fire pan with coals from this altar is mentioned is when Aaron was instructed to follow this procedure on Yom Kippur in chapter 16. These instructions were only issued after Nadab and Abihu met their fate recorded in chapter 10. Nothing is said about taking coals of fire from this altar at any other time.
"He shall take a fire pan full of coals of fire from upon the altar before the Lord and two handfuls of finely ground sweet incense, and bring it inside the veil. (Leviticus 16:12 NASB)
Besides which, if Nadab and Abihu wished to fill their fire pans with live coals, there couldn't have been any source closer that the perpetually burning fire on the altar of burnt offerings. Why would they leave the Temple premises to go find coals somewhere else when there were some so close at hand. It simply doesn't make sense.
2) Wrong Incense?
Another distinct possibility that Nadab and Abihu did not use the incense that was especially made for the Lord. This incense was considered holy and, at the risk of being cut off from the people, no one could make it in the same proportions (equal parts of specific spices) for other uses. (Exodus 30:34-38)
Again I have to wonder about this. Since a special incense was kept perpetually burning before the Lord, there had to be a large supply close at hand. Did Nadab and Abihu bring their own incense with them or...?
3) Entering The Holy of Holies
Others believe there to be another clue. The account in Leviticus 10 states that Nadab and Abihu "took their respective fire pans, and after putting fire in them, placed incense on it and offered strange fire before the Lord, which He had not commanded them".
The words "before the Lord" is often taken to mean that the brothers entered, or attempted to enter, the Holy of Holies with their fire pans of incense - something that was strictly forbidden. It is true that the Lord had made it abundantly clear that only Aaron as High Priest could enter into the Most Holy Place of the temple into His presence - and that only once a year on Yom Kippur to atone for the sins of the nation.
However, once again, I see no evidence for this restriction until Leviticus 16, that is six chapters after Nadab and Abihu were killed.
Now the Lord spoke to Moses after the death of the two sons of Aaron, when they had approached the presence of the Lord and died. The Lord said to Moses: "Tell your brother Aaron that he shall not enter at any time into the holy place inside the veil, before the mercy seat which is on the ark, or he will die; for I will appear in the cloud over the mercy seat. (Vs. 1-2 NASB)
Besides which a simple reading of this passage gives no indication that the Lord was reiterating a command He had given before.
Note: Leviticus 16 is a description of what Aaron was required to do on the Day of Atonement (You Kippur). Leviticus 23:26-32 outlines all the Feasts of Israel and how the people were to keep them. These Seven Feasts celebrated a historical event in Israel's past, but were also a prophecy of future events, four of which have already come to pass. See The Seven Feasts of Israel
The Holiness of God
Although one cannot positively rule out one (or even all three) of the above theories, at least two of them do not seem to fit into the timeline and two seem to defy common sense. Unless I have absolutely missed some crucial detail, I doubt we can be sure of what it was that the two brothers actually did, or precisely what the Bible meant when it said "strange fire". But that doesn't mean we cannot learn a valuable lesson from the events of that fateful day.
The question that arises unbidden is whether God overreacted - as some claim. But since that would contradict everything we know of our God perhaps we should examine one other possibility - the brothers infraction was just a little more complicated than disobeying a single command. What we can stake our lives on is that Nadab and Abihu showed an astounding lack of reverence and respect for the Lord - not treating Him as the holy Creator of the universe. This is made abundantly clear by the third verse.
Then Moses said to Aaron, "It is what the Lord spoke, saying, 'By those who come near Me I will be treated as holy, And before all the people I will be honored.'" So Aaron, therefore, kept silent. (Leviticus 10:3 NASB)
In my opinion, asking what precisely it was that they did to offend the Father is not the most important question simply because none of the conditions that existed when Nadab and Abihu were put to death are ever going to be repeated. There aren't, nor will there ever be altars devoted to different purposes, specially made incense, coals of fire etc.
Therefore the far more pertinent question is why?
Why did these two men, born into such a privileged position, do whatever it was they did? Having witnessed the miracles in Egypt and the amazing display of God's presence on Mt. Sinai, both brothers had first hand knowledge of the might and grandeur of the Lord. Additionally, they had to have been thoroughly familiar with all His laws, and well acquainted with everything the Lord had instructed regarding the do's and don't of Temple ritual and worship.
So why did they fall so hard?
Since it is impossible to believe that the Lord would make a random comment about something unrelated just five verses after an event of this magnitude, the clue has to lie in the following
The Lord then spoke to Aaron, saying, "Do not drink wine or strong drink, neither you nor your sons with you, when you come into the tent of meeting, so that you will not die--it is a perpetual statute throughout your generations-- and so as to make a distinction between the holy and the profane, and between the unclean and the clean, and so as to teach the sons of Israel all the statutes which the Lord has spoken to them through Moses." (Leviticus 10:8-11 NASB)
In light of the context, there is absolutely no question that the Lord was issuing a dire warning to Aaron and his other two sons that the consumption of any form of alcohol prior to entering into the Tent of meeting would not be tolerated. He made it very clear that any priest who entered the tabernacle under the influence would die.
All of which makes it hard to avoid the fact that the brothers, perhaps having had partaken just a bit too freely of the drink offering, were under the influence of strong drink at the time. To put it bluntly - they were drunk! - their judgment impaired enough that they failed to make the "distinction between the holy and the profane, and between the unclean and the clean".
As Peter said much later, shepherds who were supposed to be the spiritual leaders had to be an example to their flock. (1 Peter 5:3 NASB).
The injunction to not be "addicted" to wine was repeated several times in the New Testament..
An overseer, then, must be above reproach, the husband of one wife, temperate, prudent, respectable, hospitable, able to teach, not addicted to wine or pugnacious, but gentle, peaceable, free from the love of money. (1 Timothy 3:2-3 NASB)
Deacons likewise must be men of dignity, not double-tongued, or addicted to much wine or fond of sordid gain, (1 Timothy 3:8 NASB)
For the overseer must be above reproach as God's steward, not self-willed, not quick-tempered, not addicted to wine, not pugnacious, not fond of sordid gain, (Titus 1:7 NASB)
One should however note that the Lord never once forbade the drinking of wine, but warned against getting hooked on it. Aaron and his sons were not banned from ever consuming alcohol, but rather were told not to drink when coming into the tent of meeting! See The Bible and Alcohol
The Kindness of God
But let's return for a moment to Eleazar and Ithamar - Aaron's two younger sons.
While what happened to Nadab and Abihu was a lesson on how not to behave with God, what happened to Eleazar and Ithamar is a lesson on God's kindness and His view of legalism. After Nadab and Abihu were killed, Moses instructed Mishael and Elzaphan, the sons of Aaron's uncle Uzziel, to carry them "away from the front of the sanctuary to the outside of the camp" which they did. (Leviticus 10:4-5)
Moses' next instruction to Aaron and his sons Eleazar and Ithamar must have been particularly hard to take..
Then Moses said to , "Do not uncover your heads nor tear your clothes, so that you will not die and that He will not become wrathful against all the congregation. But your kinsmen, the whole house of Israel, shall bewail the burning which the Lord has brought about. "You shall not even go out from the doorway of the tent of meeting, or you will die; for the Lord's anointing oil is upon you." So they did according to the word of Moses. (Leviticus 10:6-7 NASB)
In other words, "they were not to display any visible signs of mourning over what had just happened. The people could mourn the transgression of their priests which had resulted in such a stunning display of God's wrath, but God's representatives, the high priest and priests, must NOT show any sign of disapproval of God's action (which mourning might be interpreted by some of the people as being). Indeed, Moses informs them that they are not even to go out of the doorway of the tent of meeting. In other words, they were to carry on with their duties within the tabernacle, finishing the offerings unto God that had been initiated, but which had been interrupted by the incident that had just occurred." 
The Grain and Wave Offering
In the previous chapter - Leviticus 9, Aaron and his sons had been instructed in regard to the grain offering some of which was to be offer up in smoke on the altar, beside the burnt offering of the morning (Vs 17). They were then told what to do with the offering of the ox and ram. Some parts of the animals were offered in smoke (the burnt offering) "But the breasts and the right thigh Aaron presented as a wave offering before the Lord, just as Moses had commanded." (Vs. 21)..
After Nadab and Abihu's bodies were removed from the sanctuary in Leviticus 10, Moses reiterated his instructions probably because, in view of their terrible loss, it was not improbably that Aaron and the other two sons would have missed something. As he said, the grain offering left over from being burned was the priest's share and was to be eaten without leaven beside the altar. The breast of the wave offering and the thigh of the heave offering were also the priest's portions of the sacrifice that could be eaten in any clean place.
However, there was a problem.
For some reason, Moses looked for the meat left over from the sin offering and found it - not in the remnants of a meal eaten by the priests (which it should have been) but in the ashes on the altar. In clear violation of a specific instruction, Eleazar and Ithamar did not eat the meat, but burned it on the altar. Moses furiously confronted his nephews as to why they also had disobeyed God's instructions.
"Why did you not eat the sin offering at the holy place? For it is most holy, and He gave it to you to bear away the guilt of the congregation, to make atonement for them before the Lord. "Behold, since its blood had not been brought inside, into the sanctuary, you should certainly have eaten it in the sanctuary, just as I commanded." (Leviticus 10:17-18 NASB)
One can very well imagine the tension of the moment when it seemed all too possible that Aaron's two surviving sons and, possibly, Aaron himself might also be struck down. Not only had they declined to exercise the privileges of the priest hood, but had intentionally violated specific instructions. It would not be much of an exaggeration to say that the very future of the Aaronic priesthood hung in the balance.
Then Aaron stepped in with a statement that was brief, to the point, and obviously heartfelt. He asked whether, in view of the tragedy that had befallen his family, could it could be good in the sight of the Lord for him to eat the sin offering as the Lord had commanded. Certainly, their actions were not normal, but then again it was no normal day.
What is truly interesting is that Aaron explanation "seemed good" in Moses' sight (Vs. 20) - he certainly seeming to sympathize with his deeply afflicted brother and was "satisfied". Even the Lord who knew the situation, took no notice of the irregularity and did not seem to be at all upset.
The difference lay in the attitude of the heart between the two sets of brothers.
In spite of the shock of Nadab and Abihu's sudden and horrific deaths, Aaron, Eleazar and Ithamar continued to fulfill their responsibilities. Although it must have been horribly difficult to not display any visible sign of grief (which could have been understood as displeasure at God's actions), they had to have been heart broken. That they managed to continue with all their other duties showed great dedication to and great reverence for God, but who can eat when heart sick?
The Letter of the Law Vs. The Spirit of The Law
There was a different dynamic at work that day - The spirit of the law took second place to the letter of the law. In fat, there is more than one instance in Scripture when the legitimate needs of God's people superseded rules.
When the hungry disciples began to pick and eat the heads of grain as they were walking through the fields one Sabbath, the Pharisees accused them of being unlawful. Jesus' reply?
But He said to them, "Have you not read what David did when he became hungry, he and his companions, how he entered the house of God, and they ate the consecrated bread, which was not lawful for him to eat nor for those with him, but for the priests alone? "Or have you not read in the Law, that on the Sabbath the priests in the temple break the Sabbath and are innocent? "But I say to you that something greater than the temple is here. (Matthew 12:3-6 NASB)
Additionally, when people's hearts were right the Lord did not even condemn an imperfect observation of the Passover. In 2 Chronicles 30 we are told of the time when king Hezekiah invited all Israel and Judah to come to Jerusalem and hold a Passover to the Lord. The letter urged the people not to be so stubborn (stiff-necked) but to return to the Lord who would not turn His face away from them if they did so.
Written invitations went out, passing from city to city through the coasts of Ephraim, Manasseh, and Zebulun. However, although the gesture was largely mocked in Israel, "some men of Asher, Manasseh and Zebulun humbled themselves and came to Jerusalem."
The people of Judah, removed the altars in Jerusalem and cast them into a brook, consecrated themselves and brought burnt offerings to the house of the Lord. However, there were many in the crowd from from Ephraim and Manasseh, Issachar and Zebulun, that had not purified themselves,
"... yet they ate the Passover otherwise than prescribed. For Hezekiah prayed for them, saying, "May the good Lord pardon everyone who prepares his heart to seek God, the Lord God of his fathers, though not according to the purification rules of the sanctuary." So the Lord heard Hezekiah and healed the people. The sons of Israel present in Jerusalem celebrated the Feast of Unleavened Bread for seven days with great joy, and the Levites and the priests praised the Lord day after day with loud instruments to the Lord . (2 Chronicles 30:18-21 NASB)
Leviticus 10 is a clear example of God judging men by their hearts, rather than their strict adherence to every detail of the ritual. And, when He did so, two were punished but the transgression of the other two was overlooked.
I find it quite telling that Nadab and Abihu were dealt with just after the Tabernacle was constructed and the Aaronic priesthood had been instituted. In the New Testament, Ananias and Sapphira were struck dead by God for selling a piece of property, then lying to the church (and God) about how much they had sold it for. (Acts 5). This occurred not long after Pentecost at the beginning of the 'church age'. In both cases there is little doubt that these incidents served to remind His people that in spite of His many kindnesses, He is still God and will be treated as such.
All too many people are under the impression that they can speak of - or to - the Lord any way they wish - that they can treat Him with arrogance, contempt or even rudeness - that they can come into His presence on their own terms. But just as Nadab and Abihu's brazenness (or sheer stupidity) cost them their lives, so too will it cost the lives of they that do not think they have to treat the Creator of this universe with the honor and respect that is due him.
The fact that Aaron didn't appear to suffer any punishment for his role in the making of the golden calf has quite a few people confused. Some believe that God was playing favorites - others that the high priests were above the law. In any event it seems like Aaron who actually made the idol was rewarded with a priesthood for himself and for his sons.
However, this is not entirely true. There are several factors to take into consideration.
When Moses descended from the mountain and saw what was going on, he stood in the gate of the camp, and said, "Whoever is for the Lord, come to me!" And all the sons of Levi gathered together to him. (Exodus 32:26 NASB). As one of the sons of Levi, Aaron must has been among those who repented.
However, the primary reason that Aaron was not destroyed was because of Moses' quite literally saved his skin. When, many years later, Moses reminded the Israelites of the incident, he said
"I fell down before the Lord, as at the first, forty days and nights; I neither ate bread nor drank water, because of all your sin which you had committed in doing what was evil in the sight of the Lord to provoke Him to anger. "For I was afraid of the anger and hot displeasure with which the Lord was wrathful against you in order to destroy you, but the Lord listened to me that time also. "The Lord was angry enough with Aaron to destroy him; so I also prayed for Aaron at the same time. (Deuteronomy 9:18-20 NASB)
While the Lord may not have destroyed Aaron at that time, I believe he paid in many ways including seeing two of his sons struck dead by the Lord. Additionally, like Moses, he never did see the promised land. He might have been the first High Priest, but he was not exempt from the decree that his generation would perish in the wilderness.
Then the Lord spoke to Moses and Aaron at Mount Hor by the border of the land of Edom, saying, "Aaron will be gathered to his people; for he shall not enter the land which I have given to the sons of Israel, because you rebelled against My command at the waters of Meribah. "Take Aaron and his son Eleazar and bring them up to Mount Hor; and strip Aaron of his garments and put them on his son Eleazar. So Aaron will be gathered to his people, and will die there." (Numbers 20:23-26 NASB)
Aaron was a very complex figure. He was used mightily of God in so many ways. He was Moses' right hand in the events leading up to the Exodus (Exodus 4:27-31) and was the first High Priest of Israel. Along with Moses he interceded for the people in Numbers 16:22 but, at the same time, he was involved in the golden calf disaster and, along with their sister Miriam, challenge Moses' sole authority (Numbers 12:1-16).
But then it may be well for us to remember that the majority of God's workers did not come from the ranks of the wise, mighty or noble. The wealthy powerful or even well educated people of the world were not the ones chosen to carry out His mission. The mighty men of the Bible, often had very humble beginnings - David was a shepherd boy, Peter a fisherman, Ruth a Gentile etc. As 1 Corinthians 1:27 says
but God has chosen the foolish things of the world to shame the wise, and God has chosen the weak things of the world to shame the things which are strong,
And there was worse, Rahab was a harlot, Jacob swindled his brother, Moses waffled at the burning bush, David's lust led to murder. Mary Magdalene was once a demon-possessed woman who became the first to see the resurrected Christ. And let us not forget that the disciples squabbled amongst themselves and fell asleep in the garden of Gethsemane.
In Aaron's case God used someone who had displayed numerous faults and failings to play a very important role.
What what all of them had in common was an enormous faith and trust in the Lord of the universe by which they not only managed to overcome the based parts of their natures, but went from being nobodies and sinners to the giants of the Bible. [PLACE IN TEXT]
 Al Maxey. Eleazar and Ithamar - An In-Depth Reflective Analysis. Reflections - Issue #270 - October 18, 2006.