“The Framers derived an independent government out of Enlightenment thinking against the grievances caused by Great Britain. Our Founders paid little heed to political beliefs about Christianity. The 1st Amendment stands as the bulkhead against an establishment of religion and at the same time insures the free expression of any belief. The Treaty of Tripoli, an instrument of the Constitution, clearly stated our non-Christian foundation. We inherited common law from Great Britain which derived from pre-Christian Saxons rather than from Biblical scripture.”
— Jim Walker, “Little-Known U.S. Document Signed By President Adams Proclaims America’s Government Is Secular,” Early America Review, Summer 1997.
In fact, it was founded on English Common Law which predated the introduction of Christianity to Britain by two hundred years.
” … [W]e know that the common law is that system of law which was introduced by the Saxons on their settlement in England, and altered from time to time by proper legislative authority from that time to the date of the Magna Charta [1215 CE], which terminates the period of the common law…and commences that of the statute law…. This settlement took place about the middle of the fifth century. But Christianity was not introduced till the seventh century…. Here, then, was a space of about two hundred years, during which the common law was in existence, and Christianity no part of it…. If, therefore, from the settlement of the Saxons to the introduction of Christianity among them, that system of religion could not be a part of the common law, because they were not yet Christians, and if, having their laws from that period to the close of the common law, we are able to find among them no such act of adoption, we may safely affirm (though contradicted by all the judges and writers on earth) that Christianity neither is, nor ever was a part of the common law. …”
— Thomas Jefferson, in a letter to Dr. Thomas Cooper, February 10, 1814. From Andrew A. Lipscomb, ed., The Writings of Thomas Jefferson , Vol. XIV, Washington, DC: The Thomas Jefferson Memorial Association, 1903, pp. 85-97. Quoted at the Ten Amendments Day site.