a·pos·tate 
(ə-pŏstāt′, -tĭt)
(ə-pŏstāt′, -tĭt)n.
One who has abandoned one's religious faith, a political party, one's principles, or a cause.
[Middle English, from Old French, from Late Latin apostata, from Greek apostatēs, from aphistanai, to revolt; see APOSTASY.]
a·postate′ adj.