We've likely already had a few.
Trump, I doubt is religious. Abraham Lincoln's religious beliefs still aren't clear.
I'd say it could happen next election or it may not happen until the 2050s or later. Too hard to tell.
An interesting question is whether there will be a Mormon or a Jewish president first.
For the sake of this discussion we're taking Presidents at their word when they state their religious views. So Trump and Obama are Christians. Did Lincoln ever state on the record whether he was a Christian or not?
https://en.wikipedia.org/wiki/Religious_views_of_Abraham_LincolnKlansmen often claimed that Lincoln was a Jew or an atheist.