Oh boy, this whole deal about America being a Christian nation really gets me going, so first thing I'm going to do is go through the religions of some of the Founding Fathers.
These are the signers of the Constitution of the United States of America:
Abraham Baldwin - Congregationalist / Episcopalian
Richard Bassett - Methodist
Gunning Bedford, Jr. - Christian Freemason / Presbyterian
John Blair - Presbyterian / Episcopalian
William Blount - Presbyterian / Episcopalian
David Brearly - Episcopalian / Christian Freemason
Jacob Broom - Lutheran / Old Swedes Church
Pierce Butler - Episcopalian
Daniel Carroll - Catholic
George Clymer - Quaker / Episcopalian
Jonathan Dayton - Presbyterian / Episcopalian
John Dickinson - Quaker / Episcopalian
William Few - Methodist
Thomas Fitzsimons - Roman Catholic
Benjamin Franklin - Presbyterian / Christian Deist (*NOTE)
Nicholas Gilman - Congregationalist
Nathaniel Gorham - Congregationalist
Alexander Hamilton - Presbyterian / Episcopalian
Jared Ingersoll - Presbyterian
Daniel of St. Thomas Jenifer - Episcopalian
William Samuel Johnson - Presbyterian / Episcopalian
Rufus King - Episcopalian / Congregationalist
John Langdon - Congregationalist
William Livingston - Presbyterian
James Madison - Episcopal
James McHenry - Presbyterian
Thomas Mifflin - Lutheran
Gouverneur Morris - Episcopalian / Non-Denominational Christian (*NOTE)
Robert Morris - Episcopalian
William Paterson - Presbyterian
Charles Cotesworth Pinckney - Episcopalian / Non-Denominational Christian (*NOTE)
Charles Pinkney - Episcopalian
George Read - Episcopalian
John Rutledge - Episcopalian
Roger Sherman - Congregationalist
Richard Dobbs Spaight - Episcopalian
George Washington - Church of England / Episcopalian
Hugh Williamson - Presbyterian
James Wilson - Presbyterian / Episcopalian
Even if one includes non-signers, you're still getting Christianity as the dominant religion.
Okay, I have (*NOTES) on some of these because wikipedia is either vague or just doesn't outright mention what denomination a person is, so I just put Non-denominational Christian (though more accurately, Protestant as they were certainly not Roman Catholic).
Franklin is a little bit of an enigma in that he seems to have gone from a more orthodox Christianity to Deism, to his own personal belief which seemed more of a liberal Christianity rather than any specific sect.
It should also be noted that Deism in the 18th century was an offshoot of Christianity that while questioning the more superstitious and mythological aspects, held to a Christian morality.
Thomas Jefferson I would argue is a Christian Deist or Unitarian (which is still rooted in Christianity), John Adams a Unitarian, Thomas Paine, probably closer to the modern definition of Deist (though he fiercely detested many Christian doctrines).
In terms of America being founded a Christian nation? Well, it depends on how you define your country. If you define America solely by its government and not its dominant culture or religion, than yeah, I suppose you could argue that it wasn't a Christian nation.
However, when you take a look at the populace being something like 90% Protestant and the general sense of ethics and culture of the people, it's very hard to say that the country wasn't Christian in its character. We had chaplains, swearing on the Bible, references to the 10 Commandments, school prayer, etc.
Furthermore, the root cause of the Pilgrims having left England was so they could practice a more biblical form of Christianity without fear of being persecuted, (which has a lot to do with the influencing the inclusion for freedom of religion in the first amendment, as well as historical significance for Americans in general).