Is the United States a “Christian Nation”? Well yes, because 73% of Americans identify as Christian (down from 86% [!] in 1990.) How about the Founders?
First, I don’t believe the Founders’ thoughts and words are inviolable sacred texts etched in stone. Even if they had declared Christianity as an animating feature of the new nation, it would not follow that we, 21st century citizens, are bound to hold to that. The fact is, the Founders never intended such a thing, and for good reason: favoring one religious belief over another would have been antithetical to the very idea of democracy.
One the principal Founders of the republic, Thomas Jefferson, famously edited a Bible by deleting all the magic from it. (He was called a “howling atheist” by his political opponents.)
Of all the animosities which have existed among mankind those which are caused by indifference of sentiments in religion appear to be the most inveterate and distressing and ought most to be deprecated. ~ George Washington
[t]he government of the United States is not in any sense founded on the Christian religion. ~ John Adams
If you want to say that the country is mostly Christian, fine. Please stop saying it was intended to be that way from the beginning.