"Thomas Jefferson said it, I believe it and that settles it." It wouldn't surprise me if that is an actual bumpersticker by now. Anything to avoid thinking for themselves. Of course, much like the fundies, these dingbats don't actually know very much about that in which they supposedly believe.
The truth is that some of the founders of the US were Christians, and the rest, except for Tom Paine and Benjamin Franklin, pretended to be when in public or writing for the public. We only know about the deistic and theistic tendencies of some of them from their private letters. Thomas Jefferson, who in many New Atheist circles seems to have taken the place of Jesus -- because so many of them so recently were Southern Baptists? Yes, probably that has a lot to do with it -- Jefferson was an Anglican deacon and, while President of the US, he led weekly prayer meetings of Congress in the Capitol Building in Washington, DC. The Virginia Statute of Religious Liberty begins: "Whereas Almighty God Hath created the mind free[...]" The "Jefferson Bible" which has recently become so famous was not meant to be seen by the public, no more than Jefferson wanted it to be publicly known that he and his slave Sally Hemings had children. Whatever private impulses may have led Jefferson to include the language about separation of church and state in his public writings and statements, he belonged to the very church from which so many Americans wanted to separate their state: the Anglican church, the church whose supreme head was the King of England. The major impulse for the separation of church and state came from Puritans in Massachusetts -- the same ones who killed all of those people in the 1690's because they thought they were witches. When the Puritans talked about freedom of religion, at least at first, they meant no more or less than freedom from the Anglican church; and the Anglican church was only created in the 1530's because Henry VIII, up until then a very loyal Catholic, wanted the freedom to divorce Anne Boleyn.
Things change, of course. The Congregationalists were Puritans in the 17th century when they fled from England and burnt witches, and today they're quite liberal on social issues as Christians go, and in 1776 and 1789 they were something in between. John Adams, born a Congregationalist, became a Unitarian, and some atheists have misunderstood this to mean that he was like a 21st-century Unitarian: either an atheist, or at the very least very friendly toward atheism. But the Unitarian Church in the 18th and 19th centuries wasn't exactly like the present-day version, and Adams wasn't its least traditionally-Christian member, just as his sometimes friend and sometimes enemy Jefferson was not the most traditionally-Christian member of the Anglican church. And it's not as if Adams was just Christian for show, like Jefferson; consider this passage from his diary of 1796:
"The Christian religion is, above all the religions that ever prevailed or existed in ancient or modern times, the religion of wisdom, virtue, equity, and humanity, let the blackguard Paine say what he will; it is resignation to God, it is goodness itself to man."
The world is complicated, and things change.
And, of course, the actual writings, both public and private, of Adams and Jefferson and many of their contemporaries, have been preserved and are quite conveniently available for the perusal of all of us in the general public. Go to any large library in the US which uses the Dewey decimal system, and go to the shelves marked 973 through 978, or to the shelves marked E in libraries using the Library of Congress system, and you will find shelves groaning with volumes of the actual written words of Jefferson, Adams, Washington, Hamilton and many of their contemporaries. Original documents from the early history of the US are all over the place.
Primary documents aren't for everybody, of course. But some of us occasionally want to appear as if we know what we're talking about.