We have all encountered people who claim that the United States was founded to be a "Christian Nation," that the founders were born-again Christians, and that any resistance to the agenda of the evangelicals trying to establish a theocracy today is thus a violation of the founders' intent. We know this is nonsense, but it's often hard to come up with convincing contrary evidence on the spot. Here's a terrific web page, provided by "Archiving Early America," with documented quotations from and references to what the founders actually said and wrote.
One excellent and useful example, from the Treaty of Tripoli, signed by President John Adams and ratified by the United States Senate in 1797: "the Government of the United States of America is not in any sense founded on the Christian religion." Can it be any clearer than that?