I first met David Barrett in 1971 in Nairobi, Kenya, where he was doing pioneering work on indigenous religious movements in Africa, a subject on which I was going to write a book, until my attention was diverted by developments in South Africa. In recent years, Barrett has been running the research office of the AD2000 Global Evangelization Movement in Rockville, Virginia. Among numbers crunchers on world religion, Barrett is at the top of his field. He publishes an annual summary in the International Bulletin of Missionary Research and edits the relevant sections of the Britannica Book of the Year and Britannica World Data. While Barrett's data are state of the art, he cautions that the art is precisely that, an art.
Estimates involve some delicate distinctions and carefully controlled guesswork.
But it is fascinating stuff. For instance, there are 1.9 billion Christians in the world and slightly over one billion Muslims. Barrett estimates that in the year 2025 there will be over three billion Christians and 1.8 billion Muslims. Not everyone is happy with the way that Barrett counts Jews. He says that in 1900 there were 12.2 million Jews, 15.5 million in 1970, 18.2 million at present, and he projects that there will be 25.5 million in the year 2025.