This is a great article by Nat Hentoff, whom I've long liked a lot ("long liked a lot"? ... if I'd had more than one cup of coffee, I'd revise that phrase. But no, I'm moving on ....) Hentoff always cuts to the chase and he knows his stuff. He explains very clearly here what the difference is between the Establishment Clause of the First Amendment and the Free Exercise Clause.
(H/T: The Common Room.)