When individuals or groups of people turn away from the principles of the gospel, they are in a state of apostasy. One example is the Great Apostasy, which occurred after the Savior established His Church. After the deaths of the Savior and His Apostles, men corrupted the principles of the gospel and made unauthorized changes in Church organization and priesthood ordinances. Because of this widespread apostasy, the Lord withdrew the authority of the priesthood from the earth. This apostasy lasted until Heavenly Father and His Beloved Son appeared to Joseph Smith in 1820 and initiated the restoration of the fulness of the gospel.
Latter-day Saints believe that, through the priesthood conferred to Joseph Smith by the ministering of angels, the authority to act in God’s name was brought back to the earth. This is “restored,” not “reformed,” Christianity. Their belief in a restored Christianity helps explain why most Latter-day Saint converts, from the 1830s to the present, converted from other Christian denominations. None of these converts thought they were leaving Christianity; they are simply grateful to learn about, and become part of, the restored Church of Jesus Christ, which they believe offers a more complete and rich Christian Church spiritually, organizationally, and doctrinally.