Unless we get a Jewish President like Sanders or a Muslim like Ellison soon, probably not until the 2030s or 2040s. The nation's growing more secular, eventually that'll result in a secular President
I'm not sure the nation "getting more secular" will result in it getting dramatically less "nominally Christian."
Not sure what nominally Christian means, or how a secular President would be evidence of the nation becoming dramatically less so, but I'll elaborate on my reasoning because I can get a little asgdsbfua sometimes.
The United States is getting more secular. By that, I mean a growing percentage of Americans are identifying as non-religious, particularly among young people. If we can assume that the sample of "people running for office" is similar to the general population, we'll see more and more secular people running for elected positions. Some of them will win. This will result in more Congresscritters, Senators, and Governors self-identifying as nonreligious. Because that is almost always the pool from which the President is selected, that will translate into a nonreligious President.
What does that mean, policy-wise? Who knows! A nonreligious President would be no better or worse than a religious one. Such a president would
probably be more socially liberal but even that isn't a guarantee. We came almost close to electing a Jew just last year, Christian Presidents are not an inevitability.