The phrase "separation of church and state" is derived from a letter written by Thomas Jefferson in 1802 to a group identifying themselves as the Danbury Baptists. In that letter, referencing the First Amendment to the United States Constitution, Jefferson writes:
"Believing with you that religion is a matter which lies solely between Man & his God, that he owes account to none other for his faith or his worship, that the legitimate powers of government reach actions only, & not opinions, I contemplate with sovereign reverence that act of the whole American people which declared that their legislature should "make no law respecting an establishment of religion, or prohibiting the free exercise thereof," thus building a wall of separation between Church & State"
First Amendment to the United States Constitution --
"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances."
Justice David Souter, writing for the majority [of the sumpreme court], concluded that "government should not prefer one religion to another, or religion to irreligion"