The seperation of church and state is a fundamental principal of the U.S. government and is perhaps the first of which to have been intentionally crafted into a secular state. The founders knew of the decades of religious wars caused by the lack of seperation and hence wanted to prevent the possibility of it in the new land later to be named the United States of America. Many of the founding fathers were indeed Deist, and it's even possible that a small fraction were atheist. I believe there may have been Christians, but they were not the majority, nor the most influential.
In their own words:
"Christianity...(has become) the most perverted system that ever shone on man. ...Rogueries, absurdities and untruths were perpetrated upon the teachings of Jesus by a large band of dupes and importers led by Paul, the first great corrupter of the teaching of Jesus."
- Thomas Jefferson
"The doctrine of the divinity of Jesus is made a convenient cover for absurdity."
- John Adams
"The Government of the United States is not in any sense founded on the Christian religion."
- Treaty of Tripoli
"Accustom a people to believe that priests and clergy can forgive sins... and you will have sins in abundance."
- Thomas Paine
"As to Jesus of Nazareth, my Opinion of whom you particularly desire, I think the System of Morals and his Religion...has received various corrupting Changes, and I have, with most of the present dissenters in England, some doubts as to his Divinity..."
- Benjamin Franklin
"Twenty times in the course of my late reading, have I been upon the point of breaking out, 'This would be the best of all possible worlds, if there were no [organized] religion in it!'"
- John Adams
And here are some articles I recommend:
http://www.stephenjaygould.org/ctrl/buckner_ncn.html
http://www.infidels.org/library/mode.../carrier2.html
And for further elaboration on the first quotation, read the following article:
http://www.geocities.com/paulntobin/paulvpeter.html
I believe that's the article I mean to refer you to. If not, you can simply browse the site of which the following article is a part of.