A person who commits or spreads the most extreme or serious heresy; the worst possible religious offender.
From arch- (supreme, chief) + heretic (a person who holds heretical beliefs). Heretic comes from Old French heretique, from Late Latin haereticus, from Greek hairetikos (able to choose).
Famous archheretics like Jan Hus or Michael Servetus were condemned more severely than ordinary heretics—they were seen as dangerously charismatic leaders spreading wrong ideas, which is why the prefix 'arch-' specifically marked them as superlatively dangerous.
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