From Greek 'hairetikos' (one who chooses) — originally neutral for choosing a philosophical school, made sinister by Christianity.
A person who holds beliefs contrary to the established doctrines of a religion; more broadly, anyone who maintains opinions at variance with what is generally accepted.
From Old French 'heretique,' from Latin 'haereticus,' from Greek 'hairetikos' (αἱρετικός), meaning able to choose, or one who chooses for oneself. The Greek word derives from 'hairein' (αἱρεῖν), to take, choose, or seize. In pre-Christian Greek, 'hairesis' (choice, school of thought) was a neutral term for any philosophical sect. Early Christians
In ancient Greek philosophy, belonging to a 'hairesis' was perfectly respectable — it simply meant you followed a particular school of thought, like the Stoics or Epicureans. The transformation of 'choice' into a crime punishable by death was entirely a product of Christian doctrinal politics.