If you search through all their writings, you can find a few of the Founding fathers who may have been atheists—some of them at certain times of their lives. Ben Franklin, for instance. But I’d say that even Ben Franklin was more of an agnostic than an atheist, most of his life.
The majority of the Founding fathers, however, were Christian. Possibly Jefferson was a Deist. But he drew upon the Christian tradition of Natural Law. The Declaration of Independence and the Constitution both are based on the position that “God” gave all men certain “inalienable rights.”
It is inconceivable that a modern atheist could come up with something like the Constitution.
Then how do you explain the quotations in private letters, by the said Founding Fathers? I’ve listed them above.
Feel free to review and reply.