Well, if what one means by that claim is that the majority of Americans are Christians, that's most likely an empirical fact: most Americans really are Christian. However, it doesn't follow that because the majority of citizens are Christian, the nation itself is Christian. Why not?
This would be an instance of the logical fallacy of composition: assuming that what's true of the parts is necessarily true of the whole, as in, most Americans are women, therefore America itself is a female nation; or most Americans are white, therefore America itself is a white nation (I hope only racists and white supremacists would be convinced by such a blatantly obviously bad argument); or most Americans are old (hello Baby Boomers!), therefore the U.S. is an old nation! The problem, in other words, is that you can't automatically transfer the properties of the parts to the whole made up of those parts.
What else could the claim that America is a Christian nation mean? Maybe that our Constitution and guiding principles are based on Judeo-Christian tenets? Well, if you compare the First Commandment and the First Amendment to the Constitution, we can put that claim to rest: while Yahweh prohibits you from worshiping any other gods (how insecure), the Bill of Rights guarantees you the right to worship any goddamn thing you please, or not to worship anything at all. You could not find two more mutually exclusive claims if you tried.
What about the Founding Fathers? Weren't they Christian? Well, I think you're starting to get the point... none of these claims is really going to hold up to a bit of rational and historical scrutiny:
God bless America? Thanks, but no need: our success is a result of our drive, our ingenuity, our ambition, our geographical luck, our historical vicissitudes, and most importantly our secular values. We are great despite being Christian...