“The makers of our Constitution were not a congregation of religious enthusiasts. If they protected religion from the intrusive propensities of all government and its functionaries, it was not because they loved churches and doctrines. It was because they loved freedom, and hated tyranny, especially tyranny over the mind. They knew that religion cannot exist except in a mind. The First Amendment protects not churches, but individuals, minds. It affirms that the inner life of individuals is none of government’s damned business.”