heresy (n.)
"an opinion of private men different from that of the catholick and orthodox church" [Johnson], c.1200, from Old French heresie (12c.), from Latin hæresis, "school of thought, philosophical sect," used by Christian writers for "unorthodox sect or doctrine," from Greek hairesis "a taking or choosing, a choice," from haireisthai "take, seize," middle voice of hairein "to choose," of unknown origin, perhaps from PIE *ser- (5) "to seize" (cognates: Hittite šaru "booty," Welsh herw "booty").

The Greek word was used in the New Testament in reference to the Sadducees, Pharisees, and even the Christians, as sects of Judaism, but in English bibles it usually is translated sect. Meaning "religious belief opposed to the orthodox doctrines of the Church" evolved in Late Latin. Transferred (non-religious) use from late 14c.