John Fea and Warren Throckmorton have good posts on David Barton's use of a "proof quote" from one of John Adams' letters to prove the "Christian America" thesis.
This is from Fea:
In the second and third paragraphs, Adams notes that the group who met in Philadelphia was so religiously diverse that the only ideas holding them together were the “general principles of Christianity.” What does he mean by this phrase? It is hard to tell at first glance. But if there were indeed “deists” and “atheists” in the room, these “general principles” must have been understood by Adams as a system of belief that was far less orthodox than the Christianity of the ancient creeds. An “atheist” might be able to find common ground around a Christian moral code (say, for example, the Sermon on the Mount), but could not affirm the existence of God. A “deist” would have rejected the Trinity, the deity of Christ, and, in some cases, God’s providence in human affairs, but he could certainly unite behind a moral code based on the teachings of Jesus. (I titled my chapter on the highly unorthodox Thomas Jefferson, “Thomas Jefferson: Follower of Jesus”). So let’s return to our original question. What did Adams mean when he said the Continental Congress was held together by “the general principles of Christianity?” If we take the beliefs of the “atheists” and the “deists” (and, I might add, the “universalists, “Socinians,” and “Preistleyans”) seriously, the “general principles of Christianity” was a phrase Adams used to describe a very vague moral code that all of these men–the orthodox and the unorthodox–could affirm.Here is a rule I follow: In general it's not a good idea to quote John Adams or Thomas Jefferson to prove the "Christian Nation" thesis. However, it's a really, really bad idea to quote the correspondence of Adams and Jefferson in the year 1813 to try to do such.
The fourth paragraph tells us that Adams believes that these “general principles” of Christianity and liberty could be easily affirmed by a host of secular writers, including Hume and Voltaire, two of the Enlightenment’s staunchest critics of organized Christianity. These “general principles of Christianity” must have been pretty watered-down if Hume and Voltaire could affirm them. Again, the reference here is to a vague morality, not the particular teachings of orthodox Christianity.