An interesting discussion on the religion of Jefferson and others has emerged. See Jason Kuznicki's post and the links contained therein. Sandefur, in commenting on all of this asserts that John Adams was a devout Christian. I'm not so sure I agree; as these posts have noted, it all depends on what it means to be "Christian."
Gregg Frazer's article at Claremont, on Theistic Rationalism, I think, correctly notes that Washington, Adams, Jefferson, Madison, & Franklin had religious views that pretty closely paralleled one another's with some nuances here and there.
I think Washington & Adams had nicer things to say about Christianity, both publicly and privately than did Jefferson & Franklin, with Madison somewhere in the middle.
John Adams was a Unitarian, which as far as I know was quite a liberal (not "orthodox") sect for the day (just as it's quite liberal today, but in a different sense), and he, like Jefferson, denied the Trinity. If you read their letters on the Trinity -- specifically commenting on Britain's 1813 repeal of their law that made it a crime to publicly deny the Trinity -- you'll see that the main difference between the two on that subject was candor, not substantive belief; Jefferson absolutely railed against the concept of the Trinity, while Adams politely remarked that he disbelieved in it.
And in explaining why he didn't believe in the Trinity, Adams lets it be known that he, like Jefferson, was a man of Reason, and did not believe in the inerrancy of Biblical Revelation.
This revelation had made it certain that two and one make three, and that one is not three nor can three be one. We can never be so certain of any prophecy, or the fulfillment of any prophecy, or of any miracle, or the design of any miracle, as we are from the revelation of nature, that is, nature’s God, that two and two are equal to four. Miracles or prophecies might frighten us out of our wits, might scare us to death, might induce us to lie, to say that we believe that two and two make five, but we should not believe it; we should know the contrary.
Had you and I been forty days with Moses on Mount Sinai, and admitted to behold the divine glory, and there been told that one was three and three one, we might not have had the courage to deny it, but we could not have believed it.
Adams reminds me a lot of Claremont; they put reason over traditional orthodoxy and Revelation, but they attempt to act as a bridge between the two and stress how more often than not, they complement one another. I think that's where you get statements from Adams, for instance, that the principles of the American government are the same as the general principles of Christianity (properly understood, of course). But if you read his letters carefully, you will see that he was a man of Reason, just like the rest.
A lot of whether someone is properly deemed a "Christian" depends of what it means to be a "Christian." Andrew Sullivan, Howard Dean, Gary Wills, Phil Donahue, Peter Jennings, Clinton & Gore, are all "professing Christians." Even I, a professed religious skeptic (but also a skeptic of dogmatic atheism) am a baptized Catholic and I suppose a "Christian" in some sense. But then ask the fundies what they think about that, and they'll shriek, "they aren't real Christians!" Well, neither was John Adams (their kind of "Christian"). At the end of this letter to Jefferson, Adams writes,
The Calvinist, the Athanasian divines ... will say I am no Christian. I say they are no Christians, and there the account is balanced.