It is right up front in the First Amendment: "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof." There is a reason both the establishment and free-exercise clauses start the Bill of Rights. If there was a single driving force behind the founding of the American colonies, it was religious liberty -- the right of non-Anglican "dissenters" to worship a they saw fit. Numerous colonies were founded on that basis. Pennsylvania expressly forbade establishment of a church.