The United States was created toward the end of a historical period known as the 18th Century Enlightenment. This was the most rational period in the history of the Western World. The West had been dominated by the Christian Church (and monarchs allied with it) from Roman times until then. And that domination had been oppressive.
The Enlightenment was time when the influence of reason waxed and the influence of Christianity waned. The Enlightenment originated in England and spread to France from which it spread much further--including to America.
In Europe, the Enlightenment was a fashion that died out with the French Revolution, but in America, where our forefathers were looking to found a nation, it did not die out. In many respects, it still exists here.
The intellectual heritage of The Enlightenment was reason and the Rights of Man.
Although Christianity did not disappear, the characteristic religion among intellectuals of the Enlightenment was Deism.
Deists believed that there was a God who started everything, gave man reason to live with, and left. Deists did accept what we today call intelligent design, but they rejected organized religions such as Christianity.
One of the biggest myths in American history is that America is the product of Christian beliefs and that the Founders were a bunch of Bible thumping "evangelical" types.
In fact, Thomas Jefferson, et al were nothing like Jerry Falwell, Pat Robertson, Dr. Dobson or others of their ilk.
The link below is to a Deism site with a lot of very interesting historical info. If you want to know the real history of America in this area, you should spend some time looking at it and at the links it provides.
http://www.deism.com/