Demography has very probably been a factor in religious history all along. The Concise Etymological Dictionary of the English Language (a favorite book of mine) tells us that our word “proletarian” derives from the Latin proles, ”offspring”: A proletarius was “a citizen of the lowest class, useful only by producing children.” One may modify this rather unsentimental description by saying that children were just about the only cherished possession of people in this class. Insofar as many intense religious movements, at least initially, appealed to poor people, this naturally gave a demographic benefit to religion. The historian Philip Jenkins has...