The last time that I checked, the Constitution defined this extremely clearly. If anything, the only point to take umbrage with is the inference that American used to be a "Christian Nation".
First of all - the identification of "Christians" as a voting block in this country is a complete non-sequitur. That group is comprised of Catholics, Protestants, Baptists, Lutherans, Evangelicals, Quakers and a whole slew of other belief systems - all of which have very few things in common - notably one of those is the inclusion of Jesus. But to imply that this group of people all believe the same thing is comical to the point of absurdity.
Secondly, the reading of the works of the Founding Fathers would tend to put a large swath of them firmly in the skeptic camp. Ben Franklin and Thomas Jefferson spring to mind as being matchless both in their contributions to a fledgling country as well as their theological doubt. Back then it was a question of trying to keep the different "Christian" sects from violently discriminating against each other.
It would seem that it took the political campaign against Roe v. Wade to galvanize all of these disparate groups into anything like a cohesive political force. A decade of appealing to this force to sponsor the modern Crusades has seemingly dredged up some of the least "turn the other cheek" kind of outspoken rhetoric that the internet has yet seen.
I find this kind of distasteful "Burn In Hell" backlash to be utterly pathetic. How anyone can call themselves a "Christian" and spit anger or hatred from their mouths at the same time is beyond my understanding. I truly wonder if these sorts of things are what people own up to during Confession or if somehow thinking, feeling and behaving in this way is considered "being a Good Christian"...