In many of the accusations of “Christian nationalism” that one hears today, the true complaint seems to be that Christianity continues to wield an outsized, or, at least, an undesirably outsized (from the accuser’s point of view), political and (especially) cultural authority. Though one may debate the matter in today’s world, the accusation is, so far as it goes in this way, historically accurate. Christianity has, in fact, been the dominant religion of the Western political and cultural world. Indeed, some might even say that one may measure the success of any given religion, defined broadly, by the extent to which it can subsume the state and the culture into its rituals, practices, strictures, beliefs, and ways of life.

A new book traces this history of politico-cultural dominance in the early medieval period: Christendom: The Triumph of a Religion, AD 300-1300 (Penguin Press), by Peter Heather.

In the fourth century AD, a new faith exploded out of Palestine. Overwhelming the paganism of Rome, and converting the Emperor Constantine in the process, it resoundingly defeated a host of other rivals. Almost a thousand years later, all of Europe was controlled by Christian rulers, and the religion, ingrained within culture and society, exercised a monolithic hold over its population. But, as Peter Heather shows in this compelling history, there was nothing inevitable about Christendom’s rise to Europe-wide dominance.

In exploring how the Christian religion became such a defining feature of the European landscape, and how a small sect of isolated congregations was transformed into a mass movement centrally directed from Rome, Heather shows how Christendom constantly battled against both so-called ‘heresies’ and other forms of belief. From the crisis that followed the collapse of the Roman Empire, which left the religion teetering on the edge of extinction, to the astonishing revolution in which the Papacy emerged as the head of a vast international corporation, Heather traces Christendom’s chameleon-like capacity for self-reinvention and willingness to mobilize well-directed force.

Christendom’s achievement was not, or not only, to define official Christianity, but – from its scholars and its lawyers, to its provincial officials and missionaries in far-flung corners of the continent – to transform it into an institution that wielded effective religious authority across nearly all of the disparate peoples of medieval Europe. This is its extraordinary story.