Apostasy
According to The Church of Jesus Christ of Latter-day Saints (LDS Church), the Great Apostasy in Christianity began not long after the ascension of Jesus Christ. It was marked with the corruption of Christian doctrine by Greek and other philosophies, with followers dividing into different ideological groups, and the martyrdom of the Apostles which lead to a loss of Priesthood authority to administer the church and its ordinances.
With all Priesthood authorities either martyred, taken from the earth, or teaching impure doctrines, there was a break in Apostolic Succession, and what remained was a mere fragment of the Church established by Jesus Christ. The Christian believers who survived the persecutions took it upon themselves to speak for God, interpret, amend or add to his doctrines and ordinances, and carry out his work without proper authority and divine direction from God. During this time, important doctrines and rites were lost or corrupted. Latter-day Saints specifically reject the early ecumenical councils for what they see as misguided human attempts to decide matters of doctrine without divine assistance, substituting debate and politics for divine revelation.
Latter-day Saints claim that various Old Testament and New Testament scriptures, including teachings of Christ himself, prophesy of this "falling away" or "apostasy." Thus, Latter-day Saints refer to the "restitution of all things" mentioned in Acts 3:20-21 and claim that a restoration of all the original and primary doctrines and rites of Christianity was necessary. They believe that important historical events such as the Protestant Reformation and the establishment of the United States Constitution, which explicitly allows for freedom of religion in its First Amendment, were necessary antecedents to the Restoration.
Read more about this topic: Restoration (Latter Day Saints)