In December 2011 the Pew Forum on Religion and Public Life (Washington) issued A Report on the Size and Distribution of the World’s Christian Population. Some of the data were developed in collaboration with the Center for the Study of Global Christianity (South Hamilton, Massachusetts). The two institutions are responsible for the bulk of reliable statistical information on religion worldwide. They are directed, respectively, by Luis Lugo and Todd Johnson. I happen to know these two gentlemen quite well. They are the religious nose counters par excellence. Ask them how many Lutherans there are in Mongolia, and how many Buddhists in Finland, they will within a few minutes come back with reasonably accurate numbers.

While the broad outline of the...