http://www.slate.com/articles/news_and_politics/faithbased/2015/07/america_is_a_country_of_converts_what_does_that_mean_for_jeb_bush_and_bobby.html

America has never elected a president who didn’t identify as Christian. To give you a sense of how important religion is to U.S. voters relative to other traits they value, a 2014 Pew survey found that an atheist candidate was significantly more of a turnoff than a gay candidate, or a candidate known to have had an extramarital affair. Still, just about every president in American history, starting with George Washington, has faced grumbling that they weren’t Christian enough.