In another wide-ranging recent book on American social class, Coming Apart: The State of White America, the political scientist Charles Murray analysed recent data on churchgoing, marriage, and related statistics to conclude that "America is coming apart at the seams. Not the seams of race or ethnicity, but of class." Most interesting of his proxies for our purposes was religion. The upper 20 per cent of the American population, data from the General Social Survey show, are considerably more likely than the lower 30 per cent to believe in God and to go to church. Among the working class, 61 per cent — a clear majority — either say they do not go to church or believe in God, or both; among the upper class, it is 42 per cent. "Despite the common belief that the white working class is the most religious group in white American society", Murray explains, "the drift from religiosity was far greater in Fishtown [his imaginary working-class community] than in Belmont [a better-off suburb]." As a headline on msnbc.com once pithily summarised research by the American sociologists W. Bradford Wilcox and Andrew Cherlin, "Who is Going to Church? Not Who You Think."

Titans of sociology such as Emile Durkheim and Max Weber understood in their own ways what most thinkers today, including the new atheists, do not — why religion might, from a secular perspective, exist in the first place. But neither they nor their contemporary heirs gave satisfactory attention to this other question: what causes it to come and go? In all likelihood, most of them did not believe it could wax as well as wane. Yet the evidence suggests that Christianity has done just that.

So if the conventional accounts have been wrong about what drives some people away from church — money, education, personal enlightenment — why are the churches of Europe as empty as they are? Why do increasing numbers of young people in the West identify themselves as "none of the above"? What is the real causal force turning a civilisation that once widely feared God into a civilisation that in some places now widely jeers at him?

The answer, I believe, has to do with a variable so seemingly humble as to have been overlooked by the titans of sociology no less than by their many descendants. That variable is the human family — more specifically, the relationship between the health of the family and the health of Christianity.