heretic (n.)

"one who holds a doctrine at variance with established or dominant standards," mid-14c., from Old French eretique (14c., Modern French hérétique), from Church Latin haereticus "of or belonging to a heresy," as a noun, "a heretic," from Greek hairetikos "able to choose" (in the New Testament, "heretical"), verbal adjective of hairein "to take" (see heresy).

[T]he heretic is not an unbeliever (far from it) but rather a man who emphasizes some point of doctrine too strongly and obsessively. [Russell Kirk, "T.S. Eliot and his Age"]