In 1779, Thomas Jefferson was concerned about the power of the Church of England within Virginia. He felt a guarantee of religious freedom was the best guarantee that America would avoid the religious intolerance and religiously inspired bloodshed that had marked much of the history of Europe. He wrote an Act for Establishing Religious Freedom; after a long battle, it became law in Virginia on 1786-JAN-16. The First Amendment of the U.S. Constitution was based in part on that act.
Thomas Jefferson, as president, wrote a letter to the Danbury Baptist Association of Connecticut on 1802-JAN-1. It contains the first known reference to the "wall of separation". The essay states in part:
"...I contemplate with solemn 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..."
LEST WE FORGET