One of the most extraordinary stories in the long history of religion is now unfolding in our own time and before our own eyes: the precipitous decline of the Christian faith in its ancient heartland, the West, over the past half-century. This decline is most obvious in Europe, where it has reached the point of being an actual collapse, but in the past decade there has been considerable evidence that a sharp decline has begun in the United States as well. There has also been that other, and parallel, extraordinary story of the revival of the Islamic faith in the Middle East over the past three or four decades. And of course, these two momentous trajectories coexist and conflict with each other—side by side, empty churches and filled mosques—in much of Western Europe itself.