The modern western world is inseparable from the idea of secularisation. From Socrates’s refusal to acknowledge the Greek gods to Copernicus’s heretical idea that the earth revolved around the sun to the French revolution’s overthrow of religious authority, the path of modernity seemed to lead away from the claims of religion. In our own time, the decline in church attendance in Europe is seen as evidence that secular modernity has entered the lives of ordinary people. Some optimistic secularists even see signs that the US, noted as a religious exception among western nations, is finally showing evidence of declining church attendance. But amid the apparent dusk of faith in Europe, one can already spot the religious owl of Minerva taking flight. This religious revival may be as profound as that which changed the course of the Roman empire in the 4th century.

In his remarkable book The Rise of Christianity, the American sociologist of religion Rodney Stark explains how an obscure sect with just 40 converts in the year 30AD became the official religion of the Roman empire by 300. The standard answer to this question is that the emperor Constantine had a vision which led to his conversion and an embrace of Christianity. Stark demonstrates the flaws in this “great man” portrait of history. Christianity, he says, expanded at the dramatic rate of 40 per cent a decade for over two centuries, and this upsurge was only partly the result of its appeal to the wider population of Hellenistic pagans. Christian demography was just as important. Unlike the pagans, Christians cared for their sick during plagues rather than abandoning them, which sharply lowered mortality. In contrast to the “macho” ethos of pagans, Christians emphasised male fidelity and marriage, which attracted a higher percentage of female converts, who in turn raised more Christian children. Moreover, adds Stark, Christians had a higher fertility rate than pagans, yielding even greater demographic advantage.

Some of the sources which Stark draws upon are open to question. What is not contestable is that many latter-day religious groups have thrived thanks to high fertility. The Mormons, for example, like Stark’s early Christians, have maintained a 40 per cent per decade population growth rate for 100 years. They remain 70 per cent of Utah’s population in the teeth of substantial non-Mormon immigration, and have even expanded into neighbouring states. In the 1980s, the Mormon fertility rate was…

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