Supreme Court, 1878 (Reynolds v. United States):
"At the first session of the first Congress the amendment now under
consideration was proposed with others by Mr. Madison. It met the views
of the advocates of religious freedom, and was adopted. Mr. Jefferson
afterwards, in reply to an address to him by a committee of the Danbury
Baptist Association, took occasion to say: "Believing with you that
religion is a matter which lies solely between man and his god; that
he owes account to noneother for his faith or his worship; that the
legislative powers of the 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." 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.
Congress was deprived of all legislative power over mere opinion, but
was left free to reach actions which were in violation of social duties
or subversive of good order."

1947 (Everson v. Board of Education):
"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 to 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[16] or professing religious beliefs or
disbeliefs, for church attendance or non-attendance. 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 between church and
State." Reynolds v. United States, supra, at 164."