The Historically Reality is that there was no single "Christianity" in the 1st Century C.E, there were a good number of competing "Christian " groups. The Epistles of John and other N.T verses make this very plain.
So there was nothing to be an Apostate from that we can view as pure or original. The Christian group that prevailed over others they considered were heretics, was the group who styled themselves as "Orthodox" , meaning "Right way".
They proceeded to corrupt Scripture, as Bart Ehrman has ably shown in his Book, and to dictate what was correct Doctrine in their view.
So this is the only late 1st Century and 2nd Century group it was possible to be an Apostate from !
And there was no continuity in the holding of doctrines and interpretations etc. in this group ! It all constantly changed.
The idea of the "Great Apostasy" is a Myth.