This introductory chapter provides an overview of Christianity in the twentieth century. The twentieth century has suffered comparative neglect at the hands of modern Western historians of Christianity, who have, on the whole, remained more interested in the intellectual and social challenges posed to the European churches in the nineteenth century. Yet it was the twentieth century that shaped the contours of the Christian faith as it is now, a culturally plural and geographically polycentric religion clustered around a number of new metropolitan loci in the non-European world. The majority of its rapidly growing number of adherents found the post-Enlightenment questions that preoccupied the churches of the North and West to be remote from their pressing everyday concerns of life and death, sickness and healing, justice and poverty. In Islamic regions of Africa and in almost all of Asia, they were also intimately concerned with the implications of living as religious minorities in a context dominated by the majority religious tradition. Their theological priorities and ethical perspectives differed accordingly from those of Christians in the North. The twentieth century thus set the agenda for the theological and ethical issues that now constitute the fault lines dividing Christians and churches from each other. The twentieth century has thus made it necessary for students of ecumenism to redraw the map of Christian unity and disunity.