To my knowledge, there are only two occurrences of the greek term ἀποστασία ('apostasies') or in the NT:
Acts 21:21 - Paul was falsely accused of teaching the Jews apostasy from Moses;
2 Thessalonians 2:2, 3 - A great apostasy foretold by Jesus, which would precede the "day of the Lord".
So, in true accuracy, only in these two stances the modern term "apostate" or "apostasy" should be used. Although the idea of falling away, or withdrawal or abandonment from faith or God o Jesus is abundant in the NT, other terms are used in the original Greek to convey that idea.
A related word 'apostasion' is used in the NT to denote a divorce or a repudiation. (Matthew 5:31; 19:7; Mark 10:4)
Eden