In his book, “The Reason for God,” Tim Keller argues that Christianity is the only truly global religion. Indeed, within a few generations of Christ, the Christian faith had spread across much of the known world—from India to North Africa to furthest reaches of the Roman Empire and into the barbarian lands of Northern Europe. Yet up until a hundred years or so ago, for all kinds of historical and sociological reasons, Christianity became a predominantly Western religion. The missionary efforts of the last hundred years began to change that, and now Pew Research predicts that Africa will be the...