apostate; renegade
Refers to a person who renounces a religious faith, political belief, or cause. Often used in historical or religious contexts.
He was once a devout believer, but later became an apostate.
Compound of 背 (hai, 'turn one's back on') + 教 (kyō, 'teaching/religion') + 者 (sha, 'person'). The exact historical derivation is uncertain, but the components clearly convey the meaning of 'one who turns away from a teaching'.