apostate (n.)

mid-14c., "one who forsakes his religion or faith," from Old French apostat and directly from Late Latin apostata (which form also was used in Middle English), from Greek apostasia, apostasis "defection, desertion, rebellion," from apostanai "to defect," literally "to stand off," from apo "off, away from" (see apo-) + stanai, aorist of histanai "to set, place," literally "cause to stand" (from PIE root *sta- "to stand, make or be firm").

The word was used from mid-14c. in non-religious situations, "one who has forsaken the party, opinion, etc., to which he previously adhered."

"unfaithful to a religious creed or to a principle," late 14c., see apostate (n.).