Forget, for a moment, what the early church did, or didn't, teach and just come back to the Bible.
In 1st john 2 v 19 we read that people who fell away (as opposed to temporary backsliding) were never truly saved in the first place.
Perhaps the warnings in the Bible, about the possibility of falling away, are given to ensure that we examine ourselves to see that we are truly saved, and not just going through the motions.