From Greek 'hairesis' (choice) — a neutral philosophical term that Christianity transformed into the gravest charge of rebellion.
Belief or opinion contrary to the orthodox doctrines of a religion; more broadly, any opinion profoundly at odds with what is generally accepted.
From Old French 'heresie,' from Latin 'haeresis,' from Greek 'hairesis' (αἵρεσις), meaning a taking, choice, or school of thought. The Greek word derives from 'hairein' (αἱρεῖν), to take or choose. In classical Greek, 'hairesis' was a neutral term for a philosophical school or sect. Its transformation into a negative term for forbidden belief occurred in early Christian usage, where choosing one's own doctrine rather than accepting the apostolic teaching became a grave offense. Key
The word 'heresy' was so feared in medieval England that it was sometimes euphemistically called 'the Lollard heresy' after John Wycliffe's followers, as though heresy itself needed to be specified to exist. The first English statute authorizing the burning of heretics — 'De Heretico Comburendo' (1401) — included 'heresy' in its very title.