Certain nonbeliever (7)

I believe the answer is:

atheist

(Other definitions for atheist that I've seen before include "One who disbelieves the existence of gods" , "I don't believe" , "One believes there are no gods" , "One denies the existence of a god" , "One who does not believe in the existence of a god" .)

I've seen this clue in the USA Today.

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