False Leaders (long article, but worth the read)
Page 7 of 55
THEOLOGICALLY APOSTATE
1 John 2:18-20 "Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; whereby we know that it is the last time. They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us. But ye have an unction from the Holy One, and ye know all things."
These antichrists weren't "sent out" as missionaries, but left to spread their perverted gospels elsewhere (Gal. 1:6-9). Christians can adhere to all sorts of different doctrines without becoming apostate, but they become apostate when they add to or delete from the gospel as revealed in the gospels of the New Testament, and teach others to do so as well.