Originally, the First Amendment applied only to the federal government, and some states continued official state religions after ratification.
Massachusetts, for example, was officially Congregational until the 1830s.
[10] In
Everson v. Board of Education (1947), the U.S. Supreme Court
incorporated the Establishment Clause (i.e., made it apply against the states):
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 to another . . . in the words of Jefferson, the [First Amendment] clause against establishment of religion by law was intended to erect 'a wall of separation between church and State' . . . That wall must be kept high and impregnable. We could not approve the slightest breach.
[11]