Now I know, they made it up.
Well, not quite, it comes from a speech by Thomas Jefferson, but it was taken completely out of context. The "wall" was, and is, a semi-permeable membrane, ideas and principles can go from church(es) to state, but no laws, or rules can go from state to church.
"Believing... that religion is a matter which lies solely between man and his God, that he owes account to none other for his faith or his worship, that the legitimate powers of government reach actions only, and 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 and State." --Thomas Jefferson to Danbury Baptists, 1802.