These aren't just individuals. Many of them are the men who actually wrote and signed the Constitution. If they say America's a Christian nation, why would you think that their statements are false? I can show what the Constitution's writers said about this country.
Show me where the Constitution supports your statement. You said it doesn't contradict it so that means it must support it.
Show it to me
I haven't even bothered to show you all the quotes from the Founding Father who completey contradict your point of view but I'll give you a couple now. Let's start with James Madison (one of the 7 key founders, chronicler of the Constitutional Convention and Father of the Constitution, Fourth President of the United States). Surely you value his opinion and he obviosly
"What influence, in fact, have ecclesiastical establishments had on society? In some instances they have been seen to erect a spiritual tyranny on the ruins of the civil authority; on many instances they have been seen upholding the thrones of political tyranny; in no instance have they been the guardians of the liberties of the people. Rulers who wish to subvert the public liberty may have found an established clergy convenient auxiliaries.
A just government, instituted to secure and perpetuate it, needs them not." --- James Madison, "A Memorial and Remonstrance", 1785
"Experience witnesseth that ecclesiastical establishments, instead of maintaining the purity and efficacy of religion, have had a contrary operation. During almost fifteen centuries has the legal establishment of Christianity been on trial. What has been its fruits? More or less, in all places, pride and indolence in the clergy; ignorance and servility in the laity; in both, superstition, bigotry and persecution." --- James Madison, "A Memorial and Remonstrance", 1785