It's plain in the first amendment to the U.S. constitution.
And read this:
http://en.wikipedia.org/wiki/Separation_of_church_and_state#United_States_of_America
Pay particular attention to this in that document:
"The clause of the First Amendment that adopted the
founders' principles of separation of church and state and freedom of religion is known as the Establishment Clause. It states, "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof...."
And this:
"The modern view adopted by the Supreme Court in the latter half of the nineteenth century is that no government -- federal, state or local -- can perform any action or make any policy which blatantly favors one faith or church over the others, or which favors belief in a God or Supreme being over non-belief."
There are some dissenters, but that's the essence of it. Anybody who thinks America was founded on Christianity is ignorant of both the nation's history and its constitution