What makes you think that?

Stuff like this, though I'm having trouble getting access to the historical poll data.

I'll believe that as soon as the next 4 presidents or so are public intellectuals, and a bunch of public intellectuals draft a new Constitution and get the states to approve it etc.

My model was that the sort of person who would become a memorable Founding Father in the 1700s is the sort of person who would become a public intellectual in the 2000s, and that atheism is more strongly linked by personal temperament than public position. I think the early American presidents were very different from the ones we have now, and so it's not clear which comparisons carve reality at the joints.

(It's not clear to me what point you would concede if an atheist president was identified.)