The word apostasia is used twice in the New Testament. In the first instance, it is of a sin alleged (Acts 21:21), where it is said of Paul that he was teaching Jews to “forsake” or “fall away from” Moses. He wasn’t really, not the way they claimed, but the allegation was that he was encouraging apostasy.
In the second instance, Paul is predicting what will happen before the day of Christ arrives. He says it this way:
“Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition . . .” (2 Thess. 2:3).
The falling away here is a forsaking that is general and visible, and will result in the manifestation of the man of sin. From the fact that this man of sin sets himself up to be worshiped in the temple of God we may infer the nature of the sin — it is a falling away from true worship into grotesque idolatry. The action of falling need not be horrendous in itself. But in this case, it is a falling away from the living God into the final death. It is a fall with no real bottom.