Great Apostasy

Great Apostasy refers to a believed falling away (apostasy) of Christians from the "true" form of their religion as taught by Jesus and the Apostles. Some Christian groups claim that a very Great Apostasy occurred during the first few centuries CE. Most commonly, it is linked to the process of Christianity becoming the state religion of the Roman Empire, which began with the legal toleration of Christianity under Constantine (allegedly the first Christian Roman Emperor), but which would soon proceed to outlawing of the earlier pagan religions. (No anti-papistry here. The ancient and venerable Christian traditions of (say) Persia or  Abyssinia do not get a look-in in this west-centric interpretation of church history. )

In its most extreme form, the idea of the Great Apostasy implies that the "true" Christian church ceased to exist in the early centuries CE, to be replaced by what was at best a very corrupted form of Christianity, or at worse a different religion. This form implies the idea of God later intervening with a new revelation to restore the Church to its original state.

Adherents
This is the idea as preached by Mormons, and also by some other less notable groups. A less extreme form of Great Apostasy is important in Seventh-Day Adventism, in some forms of Restorationism, the Catholic Apostolic Church and the New Apostolic Church. The latter two groups share with Mormonism the idea that New Testament offices (such as Apostle) became lost, and had to be restored by divine action in modern times.

In its less extreme formulations, the idea allows for a small remnant ("we're special!") who continue to adhere to the original teachings of Jesus, while more extreme forms (such as the Mormon one) do not see any such remnant surviving. Belief in such a remnant is common among Baptists, many of whom believe in the continuing existence of adherents to believer's baptism in every generation since Christ, even in those periods when paedobaptism became near universal.

Protestant view
The majority Protestant position does not believe in a Great Apostasy; although it believes that "errant" viewpoints and practices became widely accepted in the Roman Catholic and Eastern Orthodox churches, it does not consider these views or practices to be sufficiently severe so as to justify the use of the label of apostasy ("heresy" might suffice). In their mind, although the Catholic church had developed many errors, it still retained within itself the capacity to overcome those errors, as the Reformation proved—developing naturally out of the internal situation of the 16th-century Roman Catholic church rather than being imposed externally, such as by some special act of divine intervention.

Catholics and Eastern Orthodox
By contrast, Catholics and Eastern Orthodox explicitly reject the notion of any Great Apostasy. They view their current practices as the natural continuation of the teachings and practices of the early Church — not identical, but a natural evolution of them, and an attempt to apply the same principles in very different circumstances.