According to Mormon prophets, after the death of Jesus' original apostles, the Christian church gradually slipped into "the Great Apostasy"—a complete and universal abandonment of true Christian principles. Though we do not know exactly when this took place, Christian doctrine became thoroughly corrupted, and the priesthood authority necessary to administer key ordinances like baptism and the Lord's Supper was lost. The importance of believing that the true church was destroyed is underscored by Mormon apostle James Talmage: "If the alleged apostasy of the primitive Church was not a reality, The Church of Jesus Christ of Latter-day Saints is not the divine institution its name proclaims."
The restoration of true Christianity began when God the Father and Jesus Christ appeared to a youth named Joseph Smith in the spring of 1820. In this "First Vision" Jesus told Smith that all the churches were wrong and that all their creeds (statements of belief) were "an abomination." God used Smith to organize His true church again in 1830.
Jesus promised His followers that He would be with them in their ministry of making disciples, baptizing, and teaching, "even unto the end of the world" (Matt. 28:20). He promised that He would build His church, and that the "gates of hell shall not prevail against it" (Matt. 16:18). Such promises would have been broken had Jesus allowed His church to be "destroyed." While the Bible does mention that "some" shall depart from the faith (1 Tim. 4:1), it never implies that a universal or "complete" apostasy would take place prior to His return. The fact that "some [would] depart" implies that others would not. The New Testament here and elsewhere portrays apostasy as the acts and attitudes of individuals and groups breaking away from the church (see also 1 Jn. 2:19), not as the church ceasing to exist.