Apostasy', Greek: ἀποστασία - (rebellion, abandonment, state of apostasy, defection) , in Orthodox Christianity is the rejection of Christ and His saving power after baptism, a falling away. An apostate is a person who has entered into apostasy, that is, has abandoned the Church.
The Greek noun apostasia is found twice in the New Testament, (Acts 21:21; 2 Thessalonians 2:3).