Rodney Stark, a noted sociologist who studies religion, has a theory that religion as a human activity is best understood as a market or economic phenomenon. This is not to say that religion is concerned with consumption or wealth accumulation or materialism. Rather, it means that religion exists in a world where there is religious demand (the religious or spiritual behavior of people) and religion suppliers (organized faith or churches).
According to this theory, religious supply and demand act just like the supply and demand for any other good or service. The range of both religious supply and demand is wide, and in a perfect market, every person would find the exact religious supplier to meet his needs.
But just like other markets, the religion market is subject to market failure, the most notable of which is the monopoly. A state-sponsored or official religion is a religious monopoly. In a religious monopoly, since there is only one religion, the number of people who will actually have their religious demand met will be quite small. On the other hand, in a religious free market, the level of religious demand that is met will be quite high, because people can join the church or belief system that optimally meets their needs. The final bit of Stark's thesis is that there is and has always been a stable religious demand in human society. The question is always of rising or declining supply.
Stark uses his theory to explain several things. First, he says that this is the reason for low church attendance in western Europe. These countries have official religions (Church of England, Lutheranism in Scandanavia, Catholicism in the Meditteranean, etc), so most people don't have their religious needs met and thus have repressed religious behavior. He also explains why America seems unusually religious. In truth, we are no more religious than the Swedes or the Dutch. Instead, we have such a pluralistic, wide religious supply system that most people can find an outlet for their religious needs.
If this is not clear, substitute "restaurant" for "religion." There is an underlying demand for eating out. In a free market, everyone will be able to find the restaurant that meets their needs, whether they be based on price, type of food, level of service, etc. But if there were state-sponsored monopoly of, say, Kentucky Fried Chicken, then people who don't like chicken or fast food or low service quality (which would probably encompass the majority of people) would never eat out. Their restaurant demand would be repressed.
This theory makes a lot of sense to me. Economics is in one way the study of human choice-making, and religion should definitely be considered in that sphere. It also explains the many goofy oh-so-American religous expressions such as the Rapture Index (the speedometer of the end of the world), the Branch Davidians, or the Church Universal and Triumphant.
It also explains why America is an outlier in the World Values Survey--the only rich country that is not becoming increasingly secular. Finally, I think it explains what Bill McKibben calls the Christian paradox.