The current issue has an article The Prophet Motive by John Lamont. He lays out the traditional theories of religious adherence - the secularization thesis of Durkheim and Weber, then the newer rational choice (free market) model of Finke and Stark - and shows brisk contrary evidence to both. However useful each might be, they clearly don't explain everything. Lamont finds the supply-side explanation of Reginald Bibby much more powerful.
Because of the long-term exchange relations that religious organizations require, people are forever paying the costs in the here and now while most of the rewards are to be realized elsewhere and later. As a result, humans are prone to backslide, to get behind on their payments. . . . Thus, other things being equal, people will always be in favor of a modest reduction in their costs. In this fashion, humans begin to bargain with their churches for lower tension and fewer sacrifices. They usually succeed, both because it is those with the most influence—the clergy and the leading laity—who most desire to lower the level of sacrifice and because each reduction seems so small and engenders widespread approval.A good read, with much to think about, particularly in terms of the American and Western European histories of churches.
This process leads churches to lower the standards required for membership. This pleases people in the short run, but in the long run, after the effects of lowered standards make themselves felt, it drives them away.