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 argues, there was nothing inevitable about Christendom's rise to Europe-wide dominance. 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.