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Seventh-day Adventism has its roots in Adventism, a 19th-century movement that anticipated the imminent appearance (or advent) of Jesus Christ. The Adventists were also called Millerites because their group was founded by William Miller, a false prophet who predicted Jesus would return in either 1843 or 1844. When Miller’s prediction of Christ’s second coming failed to come to pass, the Millerites disbanded in dismay; this event became known as the “Great Disappointment.” But then a couple of Miller’s followers claimed to have visions to account for the failed prophecy. Instead of coming to earth, Jesus had entered the heavenly temple—thus, Miller was right, after all, they said, except his prophecy had a spiritual fulfillment instead of a physical one. One of the seers who covered for Miller was 17-year-old Ellen G. Harmon, who had her first of 2,000 purported visions in a prayer meeting shortly after Miller’s disgrace. With her vision, Ellen soon became a beacon of hope for disillusioned Millerites. She united Adventist factions and became the spiritual guide for a new religious group.
In 1846, Ellen married James White, an Adventist preacher. Soon they became convinced that Sabbath-keeping was for all Christians. In 1847, Ellen G. White had another vision—this one confirming her new belief that Sabbath-keeping was to be a primary doctrine. The Adventists under Ellen G. White’s influence became Seventh-day Adventists. Ellen G. White’s many visions and writings—she was a prolific writer—greatly shaped the doctrine of Seventh-day Adventism. Today, most Seventh-day Adventists still consider Ellen White to be a prophetess of God, even though many of her prophecies failed to come true. In fact, Seventh-day Adventists consider Revelation 19:10 (“the testimony of Jesus is the spirit of prophecy”) to be a reference to Ellen G. White’s writings.
https://www.gotquestions.org/Seventh-Day-Adventism.html