The main point of this book is that religious behavior conforms to the classic supply and demand economic model. Religions sell "products" and exact "costs." Cost/benefit determines why people join or leave religious groups.
Here is a partial summary of themes.
1) Religious belief is rational because believers evaluate the costs and benefits of their religious participation. They conclude that the benefits outweigh the costs. Therefore they are rational actors.
This is really silly. It confuses participation with belief. Yes, the perceived cost/benefit ratio may justify participation from a narrow economic point of view, but this says nothing about whether the actual beliefs are rational. Under S&F's analysis, participation in suicide cults, the Flat Earth Society, and spaceship cults is rational because the believer thought about participating and then concluded that benefits outweigh costs.
The authors also fail to recognize the equivocal nature of the term "rational." They use it in a narrow economic sense. But those who think religious belief is "irrational" do not disagree with this. They say, rather, that religious belief goes beyond the evidence, requires unwarranted, blind, assumptions, etc. and is therefore irrational. S&F do not address this conception of rationality. Accordingly, skip the first chapter. It's a waste of time.
2) Religious participation is subject to market principles of supply and demand. Religions exact "costs" for their "products" and religious participants are rational actors who perform cost/benefit analysis prior to engaging in religious activity. (Costs are such things as $$$, donations of time, willingness to undergo tension with surrounding culture, celibacy, separation from society, etc. Benefits are not usually material, but involve emotional satisfaction and often expectations of rewards after bodily death.) "High tension" groups exact more costs from their participants. "Low tension" groups exact less. A bell curve shows that the MAJORITY of people want to be in medium tension groups, those in which they feel that they are making some sacrifices, (but not too many) and that they are different from the society around them (but not too different.) When Vatican II allowed nuns to dress in civies and live in apartments, recruitment dropped because nunhood was no longer so special. Women WANTED to pay the higher cost for a distinct and visible role in society. Post Vatican II they were just social workers and teachers who had to give up sex. People WANT to pay for a good religious product. If the product is watered down, they'll go somewhere else. Likewise if the product is too expensive, as in groups that require celibacy, separation from society, large financial donations, or that advocate doctrines that subject adherents to ridicule, members will leave. Thus, medium tension churches are the most successful.
When groups change from moderate tension to low or high tension, they tend to lose members. Liberal churches in the US are shrinking. When the Unitarian Universalists recently brought back some traditional elements of the liturgy, their numbers began to grow.
I would like to have been given more information on WHY folks are attracted to medium tension groups.
3) Free religious markets result in greater levels of religious participation. The United States is "the first fully unregulated religious economy." It has the highest level of religious participation. Countries with state monopolies on religion (most of Europe, including the Western democracies) have extremely low levels of participation. Pews are empty on Sunday in Sweden. Even though these countries are democracies, they restrict the abilities of unofficial groups to operate. This book contains some shocking information about the Western "democracies" and their repression and harassment of small, unofficial churches, like Jehovah's Witnesses and Pentecostals.
Competition breeds increased religious participation because churches adjust their products to what the market wants. Also, the more supply, the greater likelihood that individuals will find a "vendor" that matches their preferences.
4) Religious preferences don't change much. Change in participation comes when the SUPPLY SIDE responds to existing demand. The US has had a 65% rate of church membership for decades, irrespective of economic conditions. When people change religions or churches, it is not because their beliefs (preferences) have changed. Rather it is because the new group better appeals to their existing preferences. (This is probably a controversial point. Do they REALLY mean that religious values don't change much?)
5) Secularization theory is all wrong. Those who say religion will die as society becomes more educated have been proven wrong. The industrialized world shows that this is not happening. Even in societies with low participation, "believing without belonging" is very high. (Western Europe has high rates of religious belief, but low levels of participation.) This section could be better. Perhaps secularization IS happening, it just doesn't manifest itself in more atheism. Perhaps god is becoming less anthropomorphic, less active, and therefore less real. This could still count as evidence of secularization in a given society. The authors don't do a good job addressing this subtle point.
Overall, this book offers a believable and coherent theory as to why religions succeed or fail, and religious believers will be disturbed to find that their purportedly sacred activities are government by crass economic forces of the Adam Smith variety. Ironically, the authors come across as very pro-religion, and pro-conservative religion to boot, even appearing subtly to offer advice to the Catholic Church on how to get its act together (though they say they don't.) Their clear preference for traditional religion is annoying and inappropriate. No need to call the Jesus Seminar an exercise in "preening silliness" or make repeated to references for the alleged vapidness of liberal theology. Their scholarship does not quite cross over into advocacy, but their sympathies are tastlessly and unprofessionally evident.
Worth reading, but you may not need to now that you've read this review.