unbeliever

From un- +‎ believer, perhaps a corruption of earlier wanbeliever (“unbeliever”).

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unbeliever (plural unbelievers)

1. One who does not believe, particularly in a deity (used by believers to describe people who do not believe in the same deity or deities as themselves).
   • 1886 October – 1887 January, H[enry] Rider Haggard, She: A History of Adventure, London: Longmans, Green, and Co., published 1887, →OCLC:

     `Well, I suppose that we must try it,' I said; and the others assented in their various ways - Leo, as though it were the best joke in the world; Job, in respectful disgust; and Mahomed, with an invocation to the Prophet, and a comprehensive curse upon all unbelievers and their ways of thought and travel.

• infidel
• non-believer
• nonbeliever
• nullifidian