Just had a brief discussion with a friend about whether or not the American founders were Christian, that made me realize that the question needs to be better defined.
If what we mean is that they were orthodox Christians, believers in the whole Bible as God's word, believers in the traditional Confessions and Creeds, at least the main names we think of as the Founders were not Christians, with at least the one exception of James Madison.
It is interesting that Madison is particularly revered by atheists when he may be the only born-again Christian among the founders. They claim him as their own because of his strong advocacy of separation of church from state, but they misunderstand: his interest was not in protecting the state from the church, but in preserving a pure church and a clear salvation message from the interference of the state.
John Adams was a Unitarian who denied the deity of Christ, Jefferson denied all the supernatural claims of the Bible, Benjamin Franklin found George Whitefield's preaching at the Great Awakening * (called at this link the "first") quite fascinating but he never gave in to the message. George Washington appears to have been more of a Mason than a Christian. It's possible he had a saving faith nevertheless, at least some claim so, but it isn't certain. We know he prayed, we know he attributed his successes in the Revolutionary War to God, and there were plenty of signs that God watched over him and his leadership.
It also depends on who is considered to have been one of the Founding Fathers. We usually think of the main names, but by some reckonings over eighty men can be counted as contributing importantly to the formation of the new nation. Among all those one would expect that at least a few of them would be true Bible-believing Christians.
But if what we mean is a basic Christian mindset, cultural Christianity, then all of them should no doubt be called Christians. If what counts is church attendance and the celebration of Christian holidays and the general observance of a Christian moral code, and an appreciation of the Christian heritage, including the Bible (not counting its supernatural revelations), then we can call the Unitarians and Deists among them Christians too. All of them belonged to a recognized Christian denomination.
So it depends on what we mean by "Christian" whether they were Christian or not. Is the point only to establish that the nation was culturally and more or less philosophically Christian? I think it wouldn't be hard to show that this is so based on the general thinking of the founding generation. Even John Locke's supposedly secular philosophy, from which the founders drew much of their inspiration, was Christian in its basic concepts. He'd studied under the Puritan John Owen. who had taught a strong line of religious tolerance between different Christian sects and denominations. Owen was a genuine Christian, whose books are still important guidance for Christians today, Locke may not have been a believer himself, but Locke certainly took much from his mentor.
In the most general sense, the philosophical sense, America can be called a Christian nation.
But it's also important to note that at least one preacher of the day felt the Constitution had ended up selling out the Christian cause. Can't remember his name at the moment.
On the side of a more genuinely believing Christian character of the nation is the fact that prayer was early on instituted in meetings of the Congress, at the suggestion of Benjamin Franklin. American government institutions have always acknowledged God since then. There was certainly a strong general sense that there is a God who is sovereign over nations, and He was honored and appealed to. This was often done "in the name of Jesus Christ" as well. Various Presidents over the years even called the nation to days of repentance, prayer and fasting for its protection. I doubt the nation could ever have been as successful as it was without that. The early public schools also used the Bible and Christian confessions and creeds as the basis for their curriculum. It's because this general acknowledgment and dependence on God has been waning, while violations of His Law have been more and more accepted, that the nation is now under Judgment. The National Cathedral today is also given over to every kind of false religion, pagan religions, anti-Christian religions, which shows the deterioration of the Christian mindset in the nation.
Whether individual American leaders were genuinely born-again believers is a separate question.
It's been a while since I've investigated all this and I don't know whether I'm going to follow it up beyond this much, but I wanted to get a few categories sorted out for any further discussion that may occur to me.
* This Wikipedia article is not the best reference. I will try to find better ones later.
Later: Here's Jonathan Edwards' description of the Great Awakening in his part of New England.