In the 4th 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 hold over its population. This was medieval Christendom: a springboard for the great eras of European colonisation and imperialism that followed. But, Peter Heather shows in this compelling new history, there was nothing inevitable about Christendom's rise and eventual dominance. In exploring how the Christian religion became such a defining feature of the European landscape, and how a small sect of isolated and intensely committed congregations was transformed into a mass movement centrally directed from Rome, Peter Heather shows how Christendom constantly battled against threats both from inside - 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 of the eleventh century onwards in which, using techniques borrowed from Roman law allied with spectacular legal forgery, the Papacy emerged as the head of a vast international corporation, Heather traces Christendom's chameleon-like capacity for self-reinvention. 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 virtually the disparate peoples of medieval Europe. This is its extraordinary story.