Please note what Scripture tells us about the 144,000 is largely reassurance that Christ will triumph in the end. After vanquishing His enemies, He will set his feet down on this earth and set up His kingdom. (See What and Where is Heaven?)
In the mean time, in the here and now there are far more pressing things that we need to focus on and pay heed to - The Wrath of God for example. Also the fact that Christians will not escape persecution - probably by the antichrist and his minions (See The Seven Seals). Additionally, chapters 17 and 18 (the 'Two Babylons') are a crucial warning dealing as they do with specific targets of the wrath of God. Considering the fact that the Lord devotes two chapters to it makes it imperative that we ensure we at least grasp the broad outlines of the message that is being given us.
In the interlude between the sixth seal and seventh seals an angel in possession of "the seal of the living God" cries out to four other angels to not harm the earth, sea, or trees until the servants of our God have been sealed.
(2) And I saw another angel ascending from the rising of the sun, having the seal of the living God; and he cried out with a loud voice to the four angels to whom it was granted to harm the earth and the sea, (3) saying, "Do not harm the earth or the sea or the trees until we have sealed the bond-servants of our God on their foreheads." (4) And I heard the number of those who were sealed, one hundred and forty-four thousand sealed from every tribe of the sons of Israel (Revelation 7:2-4 NASB)
John tells us that 144,000 people "from every tribe of the sons of Israel" are to be sealed. He goes on to say that there will be 12,000 people from each of 12 named tribes... Judah, Reuben, Gad, Asher, Naphtali, Manasseh, Simeon, Levi, Issachar, Zebulun, Joseph, and Benjamin.
See Three interesting details about this list in Footnote I.
These verses have given rise to considerable difference of opinion as to whether the 144,000 is a literal or symbolic number and who exactly are to be sealed. For example,
The Jehovah's Witnesses claim this literal number will come from their ranks, inasmuch as only 144,000 of them will gain heaven, the remainder will live on God's glorified earth.
Others believe the 144,000 will be Jews who will be saved from the physical nation of Israel.
Some traditional commentators believe that this group is symbolic of the Church.
Scripture does not support either of the first two claims, and It is beyond comprehension how 144,000 Jews could be symbolic of the Church
The Persecution of The Church and The Wrath of God.
In the book of Revelation the wrath of God is only spoken of in connection with the trumpets and the bowls, never with the Seals. Scripture makes it clear that no believer is going to be the target of God's wrath - the whole point of salvation. In fact, the breaking of the Sixth Seal is the first time that the wrath of God is mentioned. (Revelation 6:12-17) Nothing that went before had anything to do with His judgment.
However, this does not mean that the path up to this point is strewn with primroses. Much to the contrary, the Bible speaks of much trouble for Christians. Matthew 24:4-9 that describes the events leading up to and the actual persecution of the church are exactly paralleled by the first four Seals (Revelation 6-7). This means that these seals have to be referring to the persecution of Christians, not God's judgment on the rest of humanity.
See The End Of The Age Part II... The Seven Seals - A Comparison of The Seals And Matthew 24:4-9
When the Lamb broke the fifth seal John saw the souls of people under the altar, who had been slain for the word of God. The Bible indicates that the spilling of the blood of believers is the final spark that sets off the fire of God's wrath. Babylon, the great city, will be thrown down "with violence" because all the nations were deceived by her sorcery and because ...
"... in her was found the blood of prophets and of saints and of all who have been slain on the earth." (Revelation 21- 24 NASB)
The Sealed Ones
The Reason For The Sealing
Scripture tells us that the people who have been sealed (have the name of the Father "written on their foreheads" Revelation 14:1) are the only ones protected from God's wrath during the Great Tribulation. See The Two Tribulations of Matthew 24
Then out of the smoke came locusts upon the earth, and power was given them, as the scorpions of the earth have power. They were told not to hurt the grass of the earth, nor any green thing, nor any tree, but only the men who do not have the seal of God on their foreheads. (Revelation 9:3-4 NASB)
This is reminiscent of another occasion when the foreheads of men obedient to the Law were marked or symbolically 'sealed' to protect them from what was about to happen.
The Lord said to him, "Go through the midst of the city, even through the midst of Jerusalem, and put a mark on the foreheads of the men who sigh and groan over all the abominations which are being committed in its midst." But to the others He said in my hearing, "Go through the city after him and strike; do not let your eye have pity and do not spare. "Utterly slay old men, young men, maidens, little children, and women, but do not touch any man on whom is the mark; and you shall start from My sanctuary." So they started with the elders who were before the temple. (Ezekiel 9:4-6 NASB)
The Seal Itself - Literal Or Symbolic?
Scripture makes several references to God's marks or seals none of which were literal...
In Deuteronomy 6:8 Moses told God's people to keep all His statutes and His commandments, and instructing them to "bind them as a sign on your hand, and they shall be as frontlets between your eyes" (verses 2, 8). The hand and forehead probably represented the person's actions and thoughts respectively.
In Revelation 3:12 the Lord told the church at Sardis: "He who overcomes, I will make him a pillar in the temple of My God, and he shall go out no more. And I will write on him the name of My God and the name of the city of My God, the New Jerusalem...".
Referring to the time after the second coming, Revelation 22:3-4 says His bond-servants will serve Him, they will see His face, and His name will be on their foreheads.
Since God doesn't pull out His pen and write Scripture on someone's heart, nor His pocket knife to carve laws on our foreheads or in our minds, having the name of Christ or the Father written on one's forehead simply indicates allegiance and loyalty. Those who have the 'Mark of The Beast' on their foreheads will suffer the full extent of the wrath of God.
(9) Then another angel, a third one, followed them, saying with a loud voice, "If anyone worships the beast and his image, and receives a mark on his forehead or on his hand, (10) he also will drink of the wine of the wrath of God, which is mixed in full strength in the cup of His anger; and he will be tormented with fire and brimstone in the presence of the holy angels and in the presence of the Lamb. (11) "And the smoke of their torment goes up forever and ever; they have no rest day and night, those who worship the beast and his image, and whoever receives the mark of his name." (Revelation 14:9-11 NASB) (See Mark of The Beast)
As a matter of fact, when the entire scene in Revelation is symbolic... the angels holding the four winds, the great multitude arrayed in white robes etc. why should the sealing itself be literal?
Literal Hereditary Jews?
There are several compelling reasons not to consider the 144,000 to be literal hereditary Jews.
1) The New Testament teaches that all who believe in Christ are descendants of Abraham and that the middle wall of partition has been removed, making both believing Jews and Gentile into one body and one building. Paul said Abraham was
".. the father of circumcision to those who not only are of the circumcision, but who also follow in the steps of the faith of our father Abraham which he had while uncircumcised. For the promise to Abraham or to his descendants that he would be heir of the world was not through the Law, but through the righteousness of faith. (Romans 4:12-13 NASB)
Paul also wrote...
Therefore, be sure that it is those who are of faith who are sons of Abraham. (Galatians 3:7 NASB)
remember that you were at that time separate from Christ, excluded from the commonwealth of Israel, and strangers to the covenants of promise, having no hope and without God in the world. But now in Christ Jesus you who formerly were far off have been brought near by the blood of Christ. For He Himself is our peace, who made both groups into one and broke down the barrier of the dividing wall... in whom the whole building, being fitted together, is growing into a holy temple in the Lord, in whom you also are being built together into a dwelling of God in the Spirit. (Ephesians 2:12-14 and 21-22 NASB)
In light of which, are we really to believe that Jewish believers are protected while their Gentile brothers are left to perish?
2) Jesus words to the disciples in Matthew 19:28 makes it very unlikely that the 12 tribes are to be considered literally.
And Jesus said to them, "Truly I say to you, that you who have followed Me, in the regeneration when the Son of Man will sit on His glorious throne, you also shall sit upon twelve thrones, judging the twelve tribes of Israel". (Matthew 19:28 NASB)
In Moses' time, twelve men, one from each tribe were the leaders of Israel ("With you, moreover, there shall be a man of each tribe, each one head of his father's household" Numbers 1:4 NASB). However, Jesus had to be speaking figuratively because the twelve disciples did not belong to twelve separate tribes... Peter and Andrew were brothers, as were James and John.
3) Since the names of the 12 tribes are clearly listed (Judah, Reuben, Gad, Asher, Naphtali, Manasseh, Simeon, Levi, Issachar, Zebulun, Joseph, and Benjamin), a literal interpretation of this verse would mean that anyone who was not a member of these particular tribes would be left out - and that includes Abraham, Isaac, and Jacob (who never belonged to any of the tribes), Old Testament heroes such as Joshua (tribe of Ephraim) and Samson (tribe of Dan), to say nothing of all Gentiles (non-Jews). Yet, Jesus said...
"I say to you that many will come from east and west, and recline at the table with Abraham, Isaac and Jacob in the kingdom of heaven" (Matthew 8:11 NASB)
4.) In the book of Revelation, the phrase "bondservants" always refers to all of God's people, not just ethnic Jews. For example,
The Revelation of Jesus Christ, which God gave Him to show to His bond-servants, the things which must soon take place; and He sent and communicated it by His angel to His bondservant John, (Revelation 1:1 NASB)
'But I have this against you, that you tolerate the woman Jezebel, who calls herself a prophetess, and she teaches and leads My bond-servants astray so that they commit acts of immorality and eat things sacrificed to idols. (Revelation 2:20 NASB). Note: This was written to the Gentile church in Thyatira that was situated on a major road of the Roman Empire. Lydia, the seller of purple, mentioned in Acts 16:14 came from Thyatira.
And a voice came from the throne, saying, "Give praise to our God, all you His bond-servants, you who fear Him, the small and the great." (Revelation 19:5 NASB)
There will no longer be any curse; and the throne of God and of the Lamb will be in it, and His bond-servants will serve Him; (Revelation 22:3 NASB)
Given all of the above, we cannot possibly believe that the 144,000 "sealed out of every tribe of the children of Israel" are anything but a spiritual representation of the "holy nation" of believers. And, as previously mentioned, the number 144,000 itself indicates symbolism.
Do the 144,000 Have a Function?
After the listing of the 12 tribes (Revelation 7:4-8) the very next verse reads
After these things I looked, and behold, a great multitude which no one could count, from every nation and all tribes and peoples and tongues, standing before the throne and before the Lamb, clothed in white robes, and palm branches were in their hands; (Revelation 7:9 NASB)
Many mainstream commentators are of the opinion because that because the verse above comes immediately after the details of the 144,000, there has to be a cause and effect relationship between them. That is, the 144,000 are responsible for the great saved multitude - their mission apparently being to evangelize and proclaim the gospel during the tribulation period. Because of their ministry this great multitude will come to faith in Christ.
This is reading far more into the text than it actually says. All John wrote was after the sealing of the 144,000 he looked and saw this great multitude. In no shape, form, or fashion does he even hint that the multitude was evangelized by the 144,000. Besides which the expression 'after these things' is used several times in Revelation with absolutely no cause and effect between the verses. For example,
'He who has an ear, let him hear what the Spirit says to the churches.'" After these things I looked, and behold, a door standing open in heaven, and the first voice which I had heard, like the sound of a trumpet speaking with me, said, "Come up here, and I will show you what must take place after these things." (Revelation 3:22-4:1 NASB)
for the great day of their wrath has come, and who is able to stand?" After this I saw four angels standing at the four corners of the earth, holding back the four winds of the earth, so that no wind would blow on the earth or on the sea or on any tree. (Revelation 6:17 - 7:1 NASB)
"Who will not fear, O Lord, and glorify Your name? For You alone are holy; For All the nations will come and worship before You, for Your righteous acts have been revealed." After these things I looked, and the temple of the tabernacle of testimony in heaven was opened, (Revelation 15:4-5 NASB)
"The woman whom you saw is the great city, which reigns over the kings of the earth." After these things I saw another angel coming down from heaven, having great authority, and the earth was illumined with his glory. (Revelation 17:18-18:1 NASB)
"And in her was found the blood of prophets and of saints and of all who have been slain on the earth." After these things I heard something like a loud voice of a great multitude in heaven, saying, "Hallelujah! Salvation and glory and power belong to our God; (Revelation Revelation 18:24-19:1 NASB)
Revelation 14 is the only other chapter that makes mention of the 144,000 (more below). Verse 4 tells us that they have been purchased from among men as first fruits to God and to the Lamb. This has given rise to the belief that the 144,000 will be the first fruits of the coming harvest to be sealed - those who hear and heed their testimony during the Great Tribulation will also be sealed. However, the Bible make absolutely no mention of two sealings. God's clear instructions to the angels are not to harm anything until the bondservants of God are sealed. This implies that anything unsealed is harmed.
The feast of first-fruits, on the 17th day of Nisan, was a celebration of the harvest, when a sheaf representing the very first of the harvest was waved before the Lord, as a symbolic gesture that dedicated the coming harvest to Him. (This sheaf was likely to be barley, which was the first crop to ripen). Jesus rose from the dead on the Feast of First-fruits. His resurrection was like a wave offering presented to the Father as the first-fruits of the harvest to come at the end of the age. Now these are the firstfruits to God and to the Lamb.
But now Christ has been raised from the dead, the first fruits of those who are asleep. For since by a man came death, by a man also came the resurrection of the dead. For as in Adam all die, so also in Christ all will be made alive. But each in his own order: Christ the first fruits, after that those who are Christ's at His coming, (1 Corinthians 15:20-23 NASB)
Scripture doesn't tell us one way or the other whether or not the 144,000 have an assigned mission. What we need to do is stick to what the text actually says and not read our own ideas into it.
Physically or Spiritually Chaste?
In a later chapter of Revelation, these hundred forty and four thousand, who have the name of the Father on their foreheads, are depicted as standing on Mt. Zion with Christ.
(1) Then I looked, and behold, the Lamb was standing on Mount Zion, and with Him one hundred and forty-four thousand, having His name and the name of His Father written on their foreheads. (2) And I heard a voice from heaven, like the sound of many waters and like the sound of loud thunder, and the voice which I heard was like the sound of harpists playing on their harps. (3) And they *sang a new song before the throne and before the four living creatures and the elders; and no one could learn the song except the one hundred and forty-four thousand who had been purchased from the earth. (4) These are the ones who have not been defiled with women, for they have kept themselves chaste. These are the ones who follow the Lamb wherever He goes. These have been purchased from among men as first fruits to God and to the Lamb. (5) And no lie was found in their mouth; they are blameless. (Revelation 14:1-5 NASB)
Chapter 14:1-5 is a glimpse of the end of the Tribulation intended to assure the reader that Jesus will triumph in the end. However, some details are a little confusing. For example, the 144,000 are said to have "not been defiled with women" but "kept themselves chaste", which can carry one of two meanings... a) They are either physically celibate (see Matthew 19:21) or b) they are spiritually faithful.
In the Old Testament, unfaithfulness to the Father was spoken of as prostitution or adultery...
for you shall not worship any other god, for the Lord, whose name is Jealous, is a jealous God-- otherwise you might make a covenant with the inhabitants of the land and they would play the harlot with their gods and sacrifice to their gods, and someone might invite you to eat of his sacrifice, and you might take some of his daughters for your sons, and his daughters might play the harlot with their gods and cause your sons also to play the harlot with their gods. (Exodus 34:14-16 NASB)
In the New Testament, the church is known as the "bride" of the Lamb (Revelation 19:7, 21:9) who are warned not to be led astray by a "different gospel", but to stay faithful to what they had heard from the apostles (See 2 Corinthians 11:2) - a warning that we would be well advised to heed today. See The Contemporary Church
In other words, the 144.00 may well be described as celibate men to highlight their moral purity. As J. Hampton Keathley, III says...
This view fits with the following words, "these are those who follow the Lamb wherever He goes.” The whole group has remained devoted and faithful to the Lord Jesus Christ; they follow and serve Him as obedient servants in contrast to a world that as a whole goes whoring after the beast. 
Keeping Ourselves Blameless
However, what really matters is that we too need to be found blameless. As said by David Guzik in his commentary on Revelation,
It is inconceivable that a child of God could be a part of religious Babylon (though elements may creep in). But commercial Babylon, with its materialistic lure, is a constant threat to be guarded against.
In that regard a "voice from heaven" issued an unambiguous warning to God's people
I heard another voice from heaven, saying, "Come out of her, my people, so that you will not participate in her sins and receive of her plagues; for her sins have piled up as high as heaven, and God has remembered her iniquities. (Revelation 18:4-5 NASB)
It is quite clear that Revelation 17 and 18 (The Two Babylons. See link below ) summarizes the entire corrupt, greedy, and power hungry world system, both religious and commercial. The 'Great City' spoken of in both chapters is not a literal city, but a figurative description of two systems... a religious system is outlined in chapter 17, and a political/commercial system described in chapter 18. Dealing, as they do, with specific targets of the wrath of God, these chapters are a crucial, and unambiguous, warning to God's people to come out of her.
"therefore, come out from their midst and be separate (Gk. aphorizo) ," says the Lord. "and do not touch what is unclean; And I will welcome you. "And I will be a father to you, and you shall be sons and daughters to Me," says the Lord Almighty. (2 Corinthians 6:17-18 NASB)
Since it is obvious that we cannot physically leave this planet, 'coming out from among them' means separating ourselves from the world's endless quest for power, money, sex etc..
See What Is Holiness? Part II A - The Bible Defines Holiness as something set aside exclusively for God
The commandment to be separate, perfect and complete were unchanging, and consistently repeated from one end of the Bible to the other. Holiness, or separation from the world and it's standards is NOT a virtue, it is the commandment of God, and anyone who professes Christianity without holiness is as phony as the proverbial three dollar bill.
And, I suspect that the 144,000 have done just that... escaped all spiritual defilement by distancing themselves from our materialistic, godless system that has grown fat at the expense of many, the 'harlotry' of embracing of false gods, man-made religions, and the apostate church.
We have to constantly keep in mind that we are strangers and exiles on the earth. We do not belong here but are merely passing through. Being conformed to the world, or even flirting with what the world has to offer, is dangerous ground for the Christian to be treading. If you have your heart set on gaining material wealth. If everything you do revolves around 'getting ahead', then you may have a serious problem.
Note: Religious "Babylon" is a spiritual kingdom, that reached back to Shinar and the tower of Babel, then physically and/or spiritually persisted in the physical kingdom of Babylon, and other nations as diverse as Egypt, Assyria, Greece, and Rome, all of whom wreaked havoc on Israel. This kingdom, the antitheses of Jerusalem, is alive and well, and still endeavors to carry away captives and win the battle. And, until finally defeated, it will extend it's tentacles into the future when spiritual apostasy and pure unmitigated evil will peak under the regime of the beast. See Two Babylons
The Old Testament lists the tribes of Israel several different ways. We have to consider that this is done for a reason and that each variation in the lists was for a very specific reason meant to emphasize something. Three interesting details about the list of tribes in Revelation 7
1.) Levi - the priestly tribe that never had an inheritance among the twelve tribes because their inheritance was the Lord Himself (Deuteronomy 10:9) They are possibly included in the list in Revelation because all priestly functions ceased with the coming of Christ.
2.) The tribe of Dan is not mentioned possibly because they was guilty of idolatry on many occasions
The sons of Dan set up for themselves the graven image; and Jonathan, the son of Gershom, the son of Manasseh, he and his sons were priests to the tribe of the Danites until the day of the captivity of the land. (Judges 18:30 NASB)
Jeroboam set up one of his idolatrous golden calves in Dan
So the king consulted, and made two golden calves, and he said to them, "It is too much for you to go up to Jerusalem; behold your gods, O Israel, that brought you up from the land of Egypt." He set one in Bethel, and the other he put in Dan. Now this thing became a sin, for the people went to worship before the one as far as Dan. (1 Kings 12:28-30 NASB)
Others believe that 'the serpent by the way' means the Antichrist comes from the tribe of Dan
"Dan shall judge his people, As one of the tribes of Israel. "Dan shall be a serpent in the way, a horned snake in the path, That bites the horse's heels, So that his rider falls backward. (Genesis 49:16-17 NASB)
On the other hand, Dan heads the list of the tribes receiving an inheritance in Ezekiel's list (Ezekiel 48).
3.) The tribe of Joseph was usually represented by both Ephraim and Manasseh. However, while Manasseh in specifically mentioned in John's list, Ephraim is not mentioned by name but under the banner of the tribe of Joseph.
Note: Except for Ephraim and Manasseh, Israel's 12 tribes were named after Jacob's sons. When Jacob and his family moved to Egypt because of the famine in Canaan (Genesis 46) he was reunited with his youngest son Joseph whom he had long believed to be dead. Jacob took Joseph's two sons, Manasseh and Ephraim, as his own (Genesis 48:5) thus giving them a position equal to their eleven uncles. The two eventually became leaders of tribes bearing their names. (Joshua 14:4; Numbers 1 and 26.)
Joseph's second son, born in Egypt to his wife, Asenath was called "Ephraim" because "God has made me fruitful in the land of my suffering" (Genesis 41:52).
Despite Joseph's protests Jacob blessed his two grandsons, he chose to bless the younger Ephraim first and noted that he would be greater than his elder brother Manasseh (Genesis 48:5–20). However, the tribe of Ephraim was also associated with idolatry in Hosea 4:17. [PLACE IN TEXT]
 J. Hampton Keathley, III. Special Announcements (Rev 14:1-20. https://bible.org/seriespage/20-special-announcements-rev-141-20