A question for Steve

It was great to see Steve Shiffrin (and many other MOJers) at Villanova.  I appreciate Steve's report from the Scarpa conference, and will soon be posting some reactions of my own.  For now, just a quick question:  Steve says that it is "clearly right" to say that a law "founded on a religious purpose" is unconstitutional.  How, though, do we distinguish between "religious" purposes, and "purposes that many people who are religious hold"?