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The Collaborative International Dictionary
Antitheist

Antitheist \An`ti*the"ist\, n. A disbeliever in the existence of God.

Douglas Harper's Etymology Dictionary
antitheist

also anti-theist, "one opposed to belief in the existence of a god," 1813; see \n anti- + theist. Related: Antitheistic.

Wiktionary
antitheist

n. An active opponent of the belief in the existence of a god or gods.