When they were both older men, and no longer political rivals, John Adams and Thomas Jefferson began to correspond. Benjamin Rush helped bring them back together. Some of their correspondence is very helpful to understand the men, at least in later life.
One time, John Adams was ruminating on the overall thrust of American independence. Here’s what he wrote to Jefferson in 1813: “The general Principles, on which the Fathers Achieved Independence, were the only Principles in which that beautiful Assembly of young Gentlemen could Unite, and these Principles only could be intended by them in their Address, or by me in my Answer. And what were these general Principles?”
If we asked Americans today what were those general principles, we could likely hear answers along the lines of reason, of Enlightenment thinking, of “free-thought,” of skepticism when it comes to traditional religion. Is that what Adams said? No. He answered his own question: “I answer, the general Principles of Christianity, in which all those Sects were united: And the general Principles of English and American Liberty, in which all those young Men United, and which had United all Parties in America, in Majorities sufficient to assert and maintain her Independence.”
If that isn’t politically incorrect enough, John Adams goes on to assert that Christianity, and not any other philosophy or religious system, gave birth to our freedom:
Now I will avow, that I then believed, and now believe, that those general Principles of Christianity, are as eternal and immutable, as the Existence and Attributes of God; and that those Principles of Liberty, are as unalterable as human Nature and our terrestrial, mundane System. I could therefore safely say, consistently with all my then and present Information, that I believed they would never make Discoveries in contradiction to these general Principles.
Note: It’s safe to say that from the evidence we know, both Adams and Jefferson seemed to adopt a Unitarian perspective before they died. Some today would try to use that fact to negate the importance of such a statement from Adams (or other positive statements on Christianity from him or Jefferson). But we think these were complex men and John Adams’ statement stands on its own.
When we look at the sources that were influential for Jefferson as he penned the Declaration of Independence, we find indeed many instances of “the general principles of Christianity.”
 John Adams to Thomas Jefferson, June 28, 1813, in Lester J. Cappon, ed., The Adams-Jefferson Letters: The Complete Correspondence Between Thomas Jefferson and Abigail and John Adams (Chapel Hill, NC: the University of North Carolina Press, 1988), 338-340.
 John Adams to Thomas Jefferson, June 28, 1813, in Cappon, ed., 338-340.