A person who atheizes; someone who converts others to atheism or promotes atheistic ideas and beliefs.
From 'atheize' + '-er,' the suffix forming agent nouns (one who does X). This term appeared in religious polemics from the 17th-18th centuries to describe perceived threats to faith.
In medieval and Renaissance times, calling someone an 'atheizer' was basically accusing them of being a social danger—like saying they were spreading intellectual poison through the community!
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