The Establishment Clause of the First Amendment provides that “Congress shall make no law respecting an establishment of religion....”
Together with the Free Exercise Clause (“...or prohibiting the free exercise thereof…”), these two clauses—Establishment and Free Exercise—make up what are commonly known as the “Religion Clauses” of the First Amendment.
There has long been debate about the precise content and meaning of the Establishment Clause and how it interacts with the Free Exercise Clause. As the Supreme Court put it in 1947—articulating what is just as true today:
“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 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.”
Despite those words, the courts have allowed states to provide aid to religious schools and students of those schools so long as the aid does not directly support the teaching of religion and is not directed explicitly toward religious schools but aids all schools. The Court has been increasingly insistent, however, on protecting the autonomy of religious institutions from government interference or influence on internal religious governance, discipline, faith, etc.