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The Prophetic Ministry of Ellen White
Origin of The Seventh Day Adventist Church... William Miller and The Second Advent of Christ.
The Seventh Day Adventist Church’s Mission
The Seventh Day Adventist church, headquartered in Silver Spring, Maryland, boasts some 18.5 million members around the world and is said to be one of the world's fastest-growing denominations, operating in "203 out of 228 countries recognized by the United Nations". This growth is said to be primarily due to increases in Third World membership. 
While there are several foundational beliefs that distinguish Seventh day Adventists from other Christian denominations, the cornerstone of the Seventh-day Adventist Church belief system are the messages proclaimed by three angels' in Revelation 14:6-12.
The Message Of The Three Angels:
According to the web site of the General Conference of Seventh-day Adventists, "Adventists understand the three angels' messages of Revelation 14 to represent the final movement preparing the world for Christ's second coming", and see themselves as "commissioned" to present these messages to the world.  In fact, "the mission of the Seventh-day Adventist Church is to proclaim to all peoples the everlasting gospel in the context of the Three Angels' messages of Revelation 14:6-12."  In summary, Revelation 14 describes three angels who, just before Christ's return, issue three warnings to the world.
1) The first angel in verse 6, described as having the "everlasting gospel" loudly warns people to "Fear God, and give Him glory, because the hour of His judgment has come... " Although they initially thought that the first angel's message referred to God's judgment on the unredeemed, Adventists came to believe that it is an ongoing judgment of believers called "Investigative Judgment". In summary, an investigative pre-advent judgment is currently taking place in heaven which determines who is worthy to inherit God's everlasting kingdom. It is believed that this process began in 1844, a year that supposedly marked the end of the 2,300 symbolic days/years of Daniel 8:14 and is the final step before Jesus returns. (See Chapters 2 and 3)
2) The second angel in verse 8 calls out that "Babylon is fallen". According to Adventist theology, "Babylon is fallen" was a call to all Christians who attend non-Adventist churches to leave those churches (Babylon) and join the true Seventh-day Adventist remnant church. Early Advent believers said the second angel's message was fulfilled when thousands of people left their churches in 1844 to join the Advent movement.
3) The third angel's message in verse 12 is a warning not to worship the beast or receive his mark is understood as a call for people to observe the Seventh-day Sabbath. Seventh Day Adventists believe that "the great principles of God's law are embodied in the Ten Commandments... and are binding upon all people in every age".  Thus the obligation to keep the Sabbath on Saturday, is perpetual and those who disregard this commandment will face judgment and punishment. (See Chapter The Sabbath)
Conditional Immortality: Everyone ceases to exist between the time they die and are resurrected. God, who alone is immortal, will grant eternal life to His redeemed at Christ's Second Coming. The unrighteous will be resurrected after the millennium, during which time "the earth will be utterly desolate, without living human inhabitants, but occupied by Satan and his angels". 
Annihilation of the Unredeemed: Seventh-day Adventists very correctly believe Hell as a place of eternal punishment does not exist but after the thousand year period, the unrighteous dead will be resurrected, judged and sentenced to death, which is completely Biblical. Note: Some verses in the Adventist Clear Word Bible have been changed to reflect the belief that hell, as a literal fiery place of eternal punishment, does not exist. For example the word "hell', so common in other versions of the New Testament, has largely been done away with. However, few Christians seem to realize that the word "hell" does not exist in any Biblical manuscript. See Footnote I
Lifestyle: Seventh-day Adventists believe that the human body is the temple of God and should be treated as such. They, therefore, adopt the most healthful diet possible, which includes abstaining from alcohol, tobacco and drugs and the unclean foods specified in the Old Testament. Adequate rest and exercise are also emphasized. 
Note: While one can only wish more people were as concerned about their diet and lifestyle, modern SDAs health practices do not conform to everything Ellen White claimed to have been told in a vision. In fact, she said some pretty bizarre things about the consequences of not following the "light" she had received regarding her 'health reform' which, by the way, she didn't always follow herself. See Chapter VI - Ellen White and Health Reform
Finally, it is very easy to believe that most of the Seventh Day Adventist doctrines, including 'health reform' might very well have died a natural death if it hadn't been for...
The Prophetic Ministry of Ellen White:
Ellen White is an acknowledged spiritual leader of the Seventh Day Adventist church. Although she called herself a messenger of the Lord, rather than a "prophet", she knew and accepted that others believed her to be one.
Not only do Seventh Day Adventists believe Ellen G White's "expositions on any given Bible passage offer an inspired guide to the meaning of texts without exhausting their meaning or preempting the task of exegesis" , they also believe that she had the gift of prophecy. The official web site of the Seventh-day Adventist world church says "Her writings speak with prophetic authority and provide comfort, guidance, instruction, and correction to the church". 
Additionally, an article entitled 'Ellen White's role in doctrine formation' written by Ron Graybill, assistant secretary of the Ellen G. White Estate, was published in the October 1981 edition of Ministry Magazine. In it he said although "a definite distinction to be made between the normative authority of Scripture and the formative authority of Mrs. White's writings", they believed...
the revelation and inspiration of both the Bible and Ellen White's writings to be of equal quality. The superintendence of the Holy Spirit was just as careful and thorough in one case as in the other. 
Elder George I. Butler, president of the General Conference, made it extremely clear that EW's testimonies and visions were an indispensable part of the Seventh Day Adventist church. In his words,
"Our position on the Testimonies is like the keystone of the arch, take that out and there is no logical stopping-place till all the special truths of the message are gone.... Nothing is more sure than this, that this message and the visions belong together, and stand or fall together."  (Chapters 5, 6, 7, and 8, deal with different aspects of Ellen White's visions, prophecies and teachings. See index that the top of the page)
See Ellen White's pronouncements on the message of the Three Angels Footnote II
So how did all this come about?
Origin of The Seventh Day Adventist Church...
William Miller and The Second Advent of Christ.
The church traces its roots back to William Miller, a New England Baptist itinerant preacher, who was born on the 15th of February 1782, in Pittsfield, Massachusetts. Miller was a farmer who not only held the offices of deputy sheriff and justice of the peace, but also served as a captain of the 30th Infantry in the War of 1812.
In 1818, after a couple of years of intensive Bible study, Miller came to the "solemn conclusion" he was living in the final period of human history. He believed that the present world would end with the advent of Christ in 1843—1844 A. D. when "pomp, vanity, wickedness and oppression would come to an end" and "the peaceful and long desired kingdom of the Messiah would be established under the whole heaven" 
How did William Miller arrive at this conclusion?
Miller did not consider himself a prophet who had received special revelation from God, but a Bible student who based his beliefs on an analysis of specific Bible texts... Daniel 8:13–14 in particular. Note: that the Hebrew qâdôsh rendered saint in the KJV has been translated "holy one" in many other Bible versions.
Then I heard one saint (Heb. qâdôsh) speaking, and another saint said unto that certain saint (Heb. qâdôsh) which spake, How long shall be the vision concerning the daily sacrifice, and the transgression of desolation, to give both the sanctuary and the host to be trodden under foot? And he said unto me, Unto two thousand and three hundred days; then shall the sanctuary be cleansed. (Daniel 8:13-14 KJV)
In these verses Daniel overhears a holy one (presumably an angel) asking another how long the defilement of the holy place (the sanctuary) and the oppression of the host (mass of people) will continue. In verse 14 the holy one who was asked the question replied by speaking directly to Daniel telling him that the sanctuary would be restored after 2,300 evenings and mornings.
At that time, many Christians thought 'the sanctuary' was the earth that God would cleanse with fire in a final judgment on the sinners of this world.
Although the original Hebrew did not say "days" but used the words ereb (dusk) and bôqer (dawn), Miller became convinced that Daniel's "2,300 evenings and mornings" meant 2,300 years, and that the count began in 457 B.C. when Artaxerxes I of Persia issued a decree to rebuild Jerusalem. This, of course, takes one to approximately 1843- 44. Thus Miller believed that Jesus was coming to 'cleanse the sanctuary' in that period.
The movement he started, commonly referred to as the Millerite movement, "gained momentum by utilizing the latest technologies of the day, of mass communication. The high speed printing press, for example, was very important in promoting pamphlets and newsletters and newspapers and colored charts that illustrated Miller's system". 
(As an aside, note that Daniel Specified EXACTLY when the Messiah would appear. DETAILS (Scroll down slightly)
The Great Disappointment
Possibly bearing in mind Jesus' words in Matthew 24:36, William Miller did not assign an exact date to the Second Advent, but stated that it would occur about the year 1843. Article 15 of a twenty-point document issued in September 1822 said
I believe that the second coming of Jesus Christ is near, even at the door, even within twenty-one years --on or before 1843." 
As I understand it, after fellow believers asked if he could be more definite, Miller narrowed the time-period to sometime between Passover 1843 and Passover 1844. (Although the Jewish feasts do not fall on specified dates according to the Gregorian calendar, it so happened that Passover fell on March 21st in both these years.) Miller said..
"My principles in brief, are, that Jesus Christ will come again to this earth, cleanse, purify, and take possession of the same, with all the saints, sometime between March 21, 1843, and March 21, 1844." 
The movement suffered its first disappointment when March 21st 1844 came and went without Jesus appearing until Samuel Snow, a Methodist 'millerite', came to the conclusion that they had been looking at the wrong Jewish feast. The 2,300 year prophecy would not be fulfilled on Passover, but on Yom Kippur or the Day of Atonement.
He contended that Jesus would return and 'cleanse the sanctuary' on Yom Kippur (Day of Atonement) which occurs on the 10th day of Tishri or the seventh month according to the Jewish calendar. In 1844, Yom Kippur fell on October 22nd.
Note: Snow had some of his facts right. He believed that the spring/summer feasts (Passover, Unleavened Bread, First Fruits and Harvest) had been fulfilled by Jesus during His first advent, and that the three fall feasts (Trumpets, Atonement, and Tabernacles) would be fulfilled at Jesus' second coming (See The Seven Feasts of Israel)
Which bring us to the first of the Seventh Day Adventist's unique beliefs
Continue on To Chapter 2 - Investigative Judgment
Footnote I - What and Where is Hell?
The belief that Hell is a place of unending torment has been so strongly held throughout the history of Christianity that few have dared to challenge it. Besides which, since most modern challenges have come from the cults, a person who dares to question the traditional viewpoint runs the risk of being labeled a cultist. However, the deeper one delves into the subject the less persuasive the argument in favor of the traditional view become. For example, there is not a single verse in the entire Bible that says anything remotely similar to "everyone has eternal life; it is only a matter of where each will spend it." Yet, this is what most of the church believes, assuming that the idea has its origins in the Bible. It does not. Of the many references to the spirit in the Bible, never once is it said to be immortal, imperishable, or eternal. On the contrary, the Bible tells us that God alone possesses immortality. Additionally, over and over again, Christians routinely go through some astounding verbal and theological gymnastics to take the word "death", and make it mean eternal life in hell, and take the word "perish" and make it mean "never perish". However, it is totally pointless to embark on a investigation unless one is prepared to set aside one's denominational presuppositions, and other biases, and rely strictly on what the Word says. This seems to be easier said than done for many, if not most, Christians. See What and Where is Hell?
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Footnote II - Ellen White's teachings on The Three Angels' Messages
In the Great Controversy Ellen White wrote that "The first and second angel's messages were given at the right time, and accomplished the work which God designed to accomplish by them." [15 ]
The First Angel's Message
As said by Ellen White,
the first angel's message, "Fear God, and give glory to him; for the hour of his Judgment is come,” pointed to Christ's ministration in the most holy place, to the investigative Judgment, and not to the coming of Christ for the redemption of his people and the destruction of the wicked. 
Note: the first angel's message that was supposedly sounded in 1844 did not fulfill prophecy. Revelation 14:6 says the first angel had the eternal gospel "to preach to those who live on the earth, and to every nation and tribe and tongue and people". In 1844, the Gospel had not reached every single person on earth.
In fact, it still hasn't and, regardless of how much effort put in by Christians, never will.
The Second Angel's Message
According to Ellen White
As the churches refused to receive the first angel's message, they rejected the light from heaven and fell from the favor of God. They trusted to their own strength, and by opposing the first message placed themselves where they could not see the light of the second angel's message. But the beloved of God, who were oppressed, accepted the message, "Babylon is fallen," and left the churches. [17 ]
Can "Babylon" be equated with the modern church?
Revelation chapters 17 and 18 explicitly describe the fall of Babylon. Although much prophecy is not completely understood until after it has been fulfilled, the Lord devotes two chapters to what is, quite obviously, specific targets of the wrath of God. This makes it imperative that, at the very least, we grasp the broad outlines of the crucial warning we have been given.
However, it has absolutely nothing to so with abandoning one's church to join the Seventh Day Adventists.
Chapter 17 deals with the spiritual side of Babylon - a false religious system, while chapter 18 deals with Babylon as a political and commercial system. They are two faces of the same coin that operate independently of, and in opposition to, the true God. In fact, if you peer back into the mists of antiquity, the roots of spiritual apostasy go back much further than the beginning of any church - in fact, all the way back to the Tower of Babel. (Babel in Hebrew is exactly the same word often translated Babylon). See Two Babylons
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End Notes. Chapter I.. History and Beliefs
 Seventh-Day Adventism. ReligionFacts.com. 10 Nov. 2015. Web. Accessed 6 Jan. 2016.
 L. James Gibson. The three angels' messages: An Adventist imperative.
 Official website of the Seventh-day Adventist world church. Strategic Issues.
 The Official website of the Seventh-day Adventist world church. 28 Fundamental Beliefs. #19. The Law of God
 The Official website of the Seventh-day Adventist world church. 28 Fundamental Beliefs. #27. The Millennium and the End of Sin. https://www.adventist.org/fileadmin/adventist.org/files/articles/official-statements/28Beliefs-Web.pdf
 The Official website of the Seventh-day Adventist world church. Health. https://www.adventist.org/en/vitality/health/
 Official website of the Seventh-day Adventist world church. Official Statements. Methods of Bible Study.
 Official website of the Seventh-day Adventist world church. Beliefs.
 Ron Graybill. Ellen White's role in doctrine formation. Ministry Magazine. October 1981..
 G. I. Butler, president of the General Conference, cited in The Review and Herald Supplement, August 14, 1883. Messenger of the Lord, Page 426. Chapter 37 - Hermeneutics-Part 6: How Contemporaries Understood Ellen White's Authority.
 William Miller, Mr. Miller's Apology and Defence (1845).
 L. M Apology and Defence (1845). https://archichael White William Miller and the Second Great Awakening.
 Sylvester Bliss: Memoirs Of William Miller. Publisher: Andrews University Press (June 30, 2005) Pg. 77,
Also See http://www.temcat.com/L-1-adv-pioneer-lib/SBLISS/MEMOIRS.pdf. Article XV. ]
 James White. Sketches of the Christian Life and Public Labors of William Miller. Publisher: CreateSpace Independent Publishing Platform (October 13, 2014) Chapter 10. Page 133.
 The Great Controversy 1888, Page 405.
 The Great Controversy 1888, Page 423. Chapter 24—In the Holy of Holies.
 Early Writings, Page 237. The Second Angel's Message.