The following is a letter to the Editor from the 7/28/93
Chicago Tribune:

BARRINGTON--George Heinemann (Voice, July 12) argues that because
the phrase "separation between church and state" is not in the
Constitution, the principle which it embodies is likewise not part
of that document.  However, church-state separation is firmly
embedded in the Constitution.

    Because of the length of the religion clauses in the 1st
Amendment, it was inevitable that a convenient phrase would come
into use to concisely state the meaning of those clauses.  In 1802,
President Thomas Jefferson coined such a phrase, "separation between
church and state," in a letter written to some Baptists who had
asked him to explain the clauses. 

    In the Treaty of Tripoli, ratified unanimously by Congress in
1797, President George Washington wrote "...the government of the
United States is not, in any sense, founded on the Christian
religion."  After ratification, the treaty was signed by Washington's
successor, President John Adams.

    I don't think that there are any better sources on the question
of the secular founding of our country than the Constitutional
Convention of 1789, Washington, Adams, and Jefferson.