I've often heard Christians make the claim that America was founded as a "Christian nation" and that the Founding Fathers were good Christians who wanted to build their religion into the bedrock of the country. At the same time, I hear the opposite argument from secularists, that the Founding Fathers were deists at best, who explicitly rejected Christianity and deliberately kept it out of the government.
Not surprisingly, both conceptions of the Founding Fathers' belief systems (at least, the ones most responsible for actually drafting the Constitution -- Washington, Adams, Jefferson, Madison, Franklin, Wilson, G. Morris, and a few others) are incomplete at best. George Will has a good high-level overview of what the major founding fathers believed in his New York Times column today (free registration required). Here's a brief excerpt:
The founders created a distinctly modern regime, one respectful of pre-existing rights — natural rights, not creations of the regime. And in 1786, the year before the Constitutional Convention constructed the regime, Jefferson, in the preamble to the Virginia Statute for Religious Freedom, proclaimed that “our civil rights have no dependence on our religious opinions, any more than our opinions in physics or geometry.”
Ed Brayton and Jon Rowe have talked quite a bit about this over the last couple of years. They point out that deism isn't really the right way to describe most of the Founders in question, since most of them fully embraced the notion of a benevolent god who would intervene in the world from time to time, which is not compatible with true deism. On the other hand, they explicitly rejected the divinity of Christ, which certainly puts them outside the "orthodox Christianity" box as well.
Brayton, Rowe, and others prefer the term "theistic rationalism" to describe the beliefs of the key Founding Fathers. The term was coined by Gregg Frazer, a literal six-day creationist and evangelical Christian, in his doctoral thesis, and he explains it a bit here:
While there may have been a few strict Deists (meaning those who believed in a non-interventionist God, categorically rejected all revelation in favor reason, etc.) and more than a few orthodox Christians among the Founders, the key Founders -- those most responsible for the ideas upon which we declared independence and constructed the Constitution (Washington, Adams, Jefferson, Madison, Franklin, Wilson, G. Morris, and a few others) -- were neither, but rather somewhere in between. And while they may not have believed in the exact same things on religion, these key Founders were by in large agreed on certain basic tenets, which tenets put them outside both the "strict Deist" and "orthodox Christian" boxes.
Neither the "Left" nor the "Right", the secular nor the religious, can lay exclusive claim to the mantle of the Founding Fathers. Trying to coopt them is misleading at best, and downright dishonest at worst.