Excerpt:

One of the founding fathers, Thomas Jefferson, is directly responsible for giving us this phrase. In his 1802 letter to the Danbury Baptist Association, then-President Jefferson used the phrase - it was probably not the first time, but it is the most memorable one. He said:


Believing with you that religion is a matter which lies solely between man and his god, [the people, in the 1st Amendment,] 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.


Jefferson did not have a hand in the authoring of the Constitution, nor of the 1st Amendment, but he was an outspoken proponent of the separation of church and state, going back to his time as a legislator in Virginia. In 1785, Jefferson drafted a bill that was designed to squash an attempt by some to provide taxes for the purpose of furthering religious education. He wrote that such support for religion was counter to a natural right of man:


... no man shall be compelled to frequent or support any religious worship, place, or ministry whatsoever, nor shall be enforced, restrained, molested, or burthened in his body or goods, nor shall otherwise suffer, on account of his religious opinions or belief; but that all men shall be free to profess, and by argument to maintain, their opinions in matters of religion, and that the same shall in no wise diminish, enlarge, or affect their civil capacities.


Jefferson's act was passed, though not without some difficulty, in Virginia. Eyler Robert Coates wrote that the act was copied in the acts or constitutions of several states, either in words or in concepts. Jefferson himself was in France by the time word of the act reached Europe, and he wrote back to America that his act was well-thought of and admired.


Jefferson's letter specifically pointed out by the Supreme Court in Reynolds v US (98 US 145 [1878]).