In Minor v. Board of Education of Cincinnati, Judge Alphonso Taft stated religious freedom is an ideal of the United States and it's equality before the law. This includes all religious opinions and groups.
The First Amendment declares that Congress will not make laws respecting an establishment of religion. The Fourteenth Amendment says that states are not competent enough to make laws regarding this issue.
The Federal Government and states governments can't set up a church. Neither can pass laws which help or prefer one religion, help all religions, or prefer one religion over another.