The U.S. was not founded upon Christianity.
I found an interesting document on http://www.skepticfiles.org/atheist/foundr0g.htm and decided to post it here.
George Washington: "The United States is in no sense founded upon Christian Doctrine"
Thomas Jefferson: "It does me no injury for my neighbor to say there are twenty gods or no God. It neither picks my pocket nor breaks my leg."
Thomas Paine: I do not believe in the creed professed by the Jewish Church, by the Roman Church, by the Greek Church, by the Turkish Church, by the Protestant Church, nor by any Church that I know of. My own mind is my own Church.
Roger Williams: God requireth not a uniformity of religion.
Thomas Jefferson: The day will come when the mystical generation of Jesus, by the Supreme Being as his Father, in the womb of a virgin, will be classified with the fable of the generation of Minerva in the brain of Jupiter. But we may hope that the dawn of reason and the freedom of thought in these United States will do away with this artificial scaffolding, and restore to us the primitive and genuine doctrines of this most venerated Reformer of human errors.
James Madison: During almost fifteen centuries the legal establishment known as Christianity has been on trial, and what have been the fruits, more or less, in all places? These are the fruits: pride, indolence, ignorance, and arrogance in the clergy. Ignorance, arrogance, and servility in the laity, and in both clergy and laity, superstition, bigotry, and persecution.
Thomas Jefferson: I do not find in orthodox Christianity one redeeming feature.
John Adams: The divinity of Jesus is made a convenient cover for absurdity. Nowhere in the Gospels do we find a precept for Creeds, Confessions, Oaths, Doctrines, and whole carloads of other foolish trumpery that we find in Christianity.
Thomas Paine: Of all the tyrannies that affect mankind, tyranny in religion is the worst."
Abraham Lincoln: The Bible is not my Book and Christianity is not my religion. I could never give assent to the long complicated statements of Christian dogma.
Benjamin Franklin: As to Jesus of Nazareth, I think the system of Morals and his Religion, as he left them to us, the best the World ever saw or is likely to see; but I apprehend it has received various corrupting Changes, and I have, with the most of the present Dissenters in England, some doubts to his divinity.