"Religion should be subject to commonsense appraisal and rational review, as openly discussible as, say, politics, art and the weather. The First Amendment, we should recall, forbids Congress both from establishing laws designating a state religion and from abridging freedom of speech. There is no reason why we should shy away from speaking freely about religion, no reason why it should be thought impolite to debate it, especially when, as so often happens, religious folk bring it up on their own and try to impose it on others."