Apostasy. I’ve had to mention it a few times, and it’s a sad occurrence, but it does happen, unfortunately. From the Greek (because of course it is) ἀποστασία, meaning rebellion, abandonment, or defection, this refers to the rejection of Christ and His saving power of baptism - a falling away from the Orthodox Church. An apostate refers to someone who has entered into a state of apostasy, i.e., they have abandoned the Church. The noun form apostasia is found twice in the New Testament - once in Acts 21:21, and in 2nd Thessalonians 2:3. It is possible to return from apostasy, but it is a drawn out process involving the repetition of the catechumenate process, a barring from the Mysteries for usually not less than one year (but sometimes much longer), and at the end if judged ready by the priest or bishop, rechrismation. Apostasy does prevent the person from ever being tonsured or ordained as clergy, but they are otherwise restored to full communion with the Church.