This article is from the Atheism FAQ, by mathew firstname.lastname@example.org with numerous contributions by others.
"Because of the religious beliefs of the founding fathers, shouldn't
the United States be considered a Christian nation?"
Based upon the writings of several important founding fathers, it is
clear that they never intended the US to be a Christian nation. Here
are some quotes; there are more in a companion document, and the
archives at ftp.mantis.co.uk contain still more.
"What influence, in fact, have ecclesiastical establishments had on
society? In some instances they have been seen to erect a spiritual
tyranny on the ruins of the civil authority; on many instances they
have been seen upholding the thrones of political tyranny; in no
instance have they been the guardians of the liberties of the
people. Rulers who wish to subvert the public liberty may have
found an established clergy convenient auxiliaries. A just
government, instituted to secure and perpetuate it, needs them
-- James Madison, "A Memorial and Remonstrance", 1785.
"I almost shudder at the thought of alluding to the most fatal
example of the abuses of grief which the history of mankind has
preserved -- the Cross. Consider what calamities that engine of
grief has produced!"
-- John Adams, in a letter to Thomas Jefferson.
"History I believe furnishes no example of a priest-ridden people
maintaining a free civil government. This marks the lowest grade of
ignorance, of which their political as well as religious leaders
will always avail themselves for their own purpose."
-- Thomas Jefferson to Baron von Humboldt, 1813.
"I cannot conceive otherwise than that He, the Infinite Father,
expects or requires no worship or praise from us, but that He is
even infinitely above it."
-- Benjamin Franklin, from "Articles of Belief and Acts of Religion",
Nov. 20, 1728.