apostate

Etymology

From Late Latin apostata, from Ancient Greek ἀποστάτης (apostátēs, “rebel”), from ἀφίστημι (aphístēmi, “to withdraw, revolt”), from ἀπό (apó, “from”) + ἵστημι (hístēmi, “to stand”).

adj

1. Guilty of apostasy.
   We must punish this apostate priest.
   An enormous number of monks and priests gave themselves up to sorcery, and there was no lack of apostate priests to perform the ceremonies of the Black Mass. 1936, Rollo Ahmed, The Black Art, London: Long, page 101

noun

1. A person who has renounced a religion or faith.
2. (Roman Catholicism) One who, after having received sacred orders, renounces his clerical profession.
3. (by extension) One who has renounced a political party, a cause, etc.