An atheist president is one of those thing that's unthinkable until it isn't. Like a Black president or nationwide gay marriage. Seriously, how many people who have predicted those things in the yer 2000?
And to all the people saying America is too religious, Chile has an agnostic president.
A black president in 2000 was not an unrealistic possibility.
An atheist president isn't impossible, but it would have to be a matter of someone who is really well liked, and then people find out he's an atheist and they are willing to overlook it.
(And I say he as I think a woman atheist president might be a bit much.)