Addressed to the Danbury, Connecticut, Baptist Association, Jefferson's letter stated, in part:
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 [the First Amendment] 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. The importance of this letter can only be grasped in its historical context, in its influence on the U.S. Supreme Court's rulings from then through the bicentennial we now commemorate, and on what the present Supreme Court will make of it between now and Independence Day, 2002.
Jefferson's "wall of separation" metaphor was employed by the Supreme Court in 1879 in its first religious liberty case, Reynolds v. United States. Citing the Jefferson quote above, the Court held that "coming as this does from an acknowledged leader of the advocates of the measure, it may be accepted almost as an authoritative declaration of the scope and effect of the amendment thus secured."
The next time the High Court utilized "the wall" was in the landmark 1947 case, Everson v. Board of Education. The Court stated, in Justice Hugo Black's ringing words:
The "establishment of religion" clause of the First Amendment means at least this: Neither a state nor the Federal Government can set up a church. Neither can pass laws which aid one religion, aid all religions, or prefer one religion over another. Neither can force nor influence a person to go to or remain away from church against his will or force him to profess a belief or disbelief in any religion. No person can be punished for entertaining or professing religious beliefs or disbeliefs, for church attendance or nonattendance. No tax in any amount, large or small, can be levied to support any religious activities or institutions, whatever they may be called, or whatever form they may adopt to teach or practice religion. Neither a state nor the Federal Government can, openly or secretly, participate in the affairs of any religious organizations or groups and vice versa. In the words of Jefferson, the clause against establishment of religion by law was intended to erect "a wall of separation...