Claiming that the U.S. wasn't a Christian nation is as absurd as claiming that the U.S. was a Christian nation. The U.S. was and is a tax farm; a slavers' empire; a social construct for the betterment of some plutocrats; a way to distract Great Britain from a re-arming France for long enough to make later Napoleons sufficiently deadly. The same wealthy bankers and person-owners who edited their own personal Bibles and didn't turn their other cheek to the British would have been willing nonetheless to identify as Christians, deists, Quakers, Puritans, frontiersmen, farmers, gentlemen, Americans, Portuguese, or any other damn thing they could come up with to justify directing tax revenues to Philadelphia instead of to London.
What is a nation? The true believers? The true skeptics? The handful of one-percenters in tall socks and wigs who contradicted themselves both extemporaneously and over the course of years? Was or was not Cotton Mather genuine, and should things have been left to King George which were King George's, rather than having been given to President George? Conversely, are the things rendered unto Caesar his temporal authority, or his eternal damnation, and is he an icon or a humble servant? If the slaves were mostly Baptists, was America Christian? How about we compromise--3/5 of the slaves' amalgamated Christianity can be counted for the total, resulting in 50.8% retroactive Christianity, making the nation undeniably Christian, although the Constitution, or was it the Declaration, or maybe the etching out front of the Lincoln Memorial, mentioned a Creator but not a Savior, thereby guaranteeing that no one comes to the Father except by avoiding the Declaration and going through the Savior instead?