Jefferson's Wall of Separation Letter
Thomas Jefferson wrote a letter to the Danbury Baptist Association in
1802 to answer a letter from them, asking why he would not proclaim
national days of fasting and thanksiving, as had been done by Washington
and Adams before him. The letter contains the phrase "wall of separation
between church and state," which lead to the short-hand for the
Establishment Clause that we use today: "Separation of church and
state."
The letter was the subject of intense scrutiny by Jefferson, and he
consulted a couple of New England politicians to assure that his words
would not offend while still conveying his message: it was not the place
of the Congress or the Executive to do anything that might be
misconstrued as the establishment of religion.
http://www.constitution.org/tj/sep_church_state.txt
