A person who actively opposes theism and belief in God, going beyond simple atheism to actively argue against religion.
From anti- (against) + theist (one who believes in God). This word emerged in the 19th century to distinguish those who actively fight against religion from those who simply don't believe.
While atheists simply don't believe in God, antitheists actively work against religious belief—think of famous skeptics like Christopher Hitchens who spent careers arguing that religion is harmful, not just false.
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