Things I learned from the Randi Rhodes show
In 1796 the United States signed a treaty with Tripoli (modern day Libya) that specifically stated that the United States was not a Christian nation. Apparently this was necessary because Tripoli, being Muslim, balked at signing a treaty with a Christian nation.
Here's the relevent article:
As the government of the United States of America is not in any sense founded on the Christian Religion,-as it has in itself no character of enmity against the laws, religion or tranquility of Musselmen,-and as the said States never have entered into any war or act of hostility against any Mehomitan nation, it is declared by the parties that no pretext arising from religious opinions shall ever produce an interruption of the harmony existing between the two countries.
This treaty was submitted by John Adams and ratified by the U.S. Senate, thus making it the law of the land.
More info here.
Learn something new every day!