The Pew Research Center recently asked voters whether certain characteristics would make it more or less likely that a politician would get their votes. For example, if the candidate were a woman, 19% of those surveyed said they would be more inclined to vote for her while 9% said less, for a net positive of 10%.
So Pew ranked the characteristics from highest net positive traits (served in the military, was/is a governor) to the highest net negative (take a guess):

There we are, way at the bottom. 48% of voters, on the whole, say they would be less likely to vote for someone who was an atheist. Having an affair would hurt you lessthan admitting there’s no evidence for the existence of God. Someone who’s never held elected office would be at an advantage over someone who didn’t believe in fairy tales.