I think people are grossly misinformed when they say the Founding Fathers were Christians, and wanted to establish a Christian nation. There are a few instances of quotes that would leave you to believe that it is Christian nation, but the Founding Fathers themselves tended to waffle on the theory, based on Election season.
However, some astounding circumstantial evidence does exist.
Many of these Founding Fathers were deists, which is not the same as modern day Christiandom or Christianity at the time. For example, Thomas Jefferson was so out of sync with the existing sects of Christianity that he practiced his own variant of it. He even wrote his own bible.
Secondly, there is a very clear legislative basis for the fact that this is not a Christian Nation. The first is of course our fundamental right to the separation of Church and State. Jefferson made some great quotes on this.
The second legislative evidence comes from the Treaty of Tripoli, Article 11. Wiki it. Keep in mind, that this was ratified in 1797 by many of the original members of Congress, and was signed into law by John Adams. You can look up to see who was on Congress at the time, but I think it serves to show that this treaty was ratified unanimously, and Adams did not decide to veto it.
Essentially Article 11 states, in no uncertain terms whatsoever, that America is not a Christian Nation, was never intended to be such.