Religion has been waning in influence for several centuries, especially in Europe and North America. There have been a few brief and local revivals, but in recent years the pace of decline has accelerated.

Today one of the largest categories of religious affiliation in the world—with more than a billion people—is no religion at all, the “Nones.” One out of six Americans is already a None; by 2050, the figure will be one out of four, according to a new Pew Research Center study. Churches are being closed by the hundreds, deconsecrated...