We are NOT a Christian nation, in that Christianity is NOT the official state religion of the US. Christianity is the majority religion, in that most Americans claim to be Christian. Because Christianity is the majority religion, some Christians think they are owed a certain amount of privelege (just my opinion), and some of them argue that the US was founded on Christian principles rather than Theism.
We could end up a Christian theocracy, a truly Christian nation, if Dominionists like Sarah Palin have their way.
1. First I agree that many of the founders were Deist, or believed in a God of nature not just the divine laws (Thomas Jefferson even reedited his own version of the Bible deleting any references to miracles and just focusing on the natural laws for humanity's sake). Many Christian groups do not consider the Free Masons or the Rosecrutians and other spiritualist followings to be Christian but even satanic cults.
However, the fact that these founders "gathered in prayer" and prayed to God for guidance to write down our founding documents -- THAT is a Christian concept to agree "in prayer in Christ" in order to receive and invoke God's will so that decisions made are for the greater good of posterity, not just our own thoughts or efforts in the present.
2. Also, if you look at the early Quakers, from William Penn to the First Ladies such as Dolly Madison, there are equal influences from the liberal Christian Left who balanced the Christian Right. So the Christian Right is not the only representation of Christianity in the founding traditions of this nation. If you include the left, this is where the push for inclusion and tolerance and nonviolence instead of war, even abolition of slavery comes from.
3. Another Christian concept is the respect for civil authority and law, even if you disagree with it. If we didn't have that strong Christian inluence and tradition in this country, I doubt we would survive the tumultuous changes in policy that would otherwise divide the nation worse than has actually occurred.
It is one thing to believe in God, and to have power to represent or even impose laws or judgment in the name of this God. But faith in God alone by Deists and Theists is not enough to keep this country as stable and productive as it has been.
Over and over again, despite injustices and political conflicts and abuses, I have seen people apply their "Christian faith" to respecting and enduring the democratic process of reforms, no matter how much persecution and suffering this entails in "delaying justice" instead of taking justice into their own hands.
Whether you call that invoking faith or trust in higher Justice or invoking Jesus, that is still the meaning of Christ for the people to embody and follow the laws by conscience.
That is different from just being Deist or Theist.