This article examines the link between economics and religion. If you want to avoid an argument over religion at your next dinner party, you might suppose it safe to invite an economist or two. They, of all people, could be expected to stick to Mammon. Or maybe not, if a new paper by Robert Barro, one of America's best-known economists, and Rachel McCleary, a colleague at Harvard University, is any guide. Recently, Niall Ferguson, a British historian at New York University, argued that today's economic stagnation in Germany and other European countries owes much to the decline of religious belief and church attendance during the past four decades. More prosperous countries seem to have lower rates of church attendance, although America--the best instance of a country of competing sects rather than a state religion--is a conspicuous exception. If there is a link between religion and economic performance, then economists ought to have something to say about it.