“Believing with you 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 legislative 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, in a Letter to a Committee of the Danbury Baptist Association, Connecticut  (January 1, 1802)

Please know, this is not in the Constitution.  It is in a letter to a group of baptists who were concerned that the Federal government would attempt to make the Congregationalist Church the American national church.  It was a letter to assure these Baptists that they were protected from the Federal Government intruding on their worship and beliefs not to keep Christianity out of the public square and government.