The separation of church and state is a legal and political principle derived from the First Amendment to the United States Constitution, which reads, "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof . . ." The phrase "separation of church and state" is generally traced to an 1802 letter by Thomas Jefferson to the Danbury Baptists, where Jefferson spoke of the combined effect of the Establishment Clause and the Free Exercise Clause of the First Amendment. It has since been quoted in several opinions handed down by the United States Supreme Court.[1]

http://en.wikipedia.org/wiki/Separat..._United_States