In its December 2007 issue (p. 26 or so), National Geographic reports on World Religions (with helpful and colorful diagrams.) The world breaks down this way: 33% Christian, 21% Muslim, 14% Nonbelievers, 13% Hindus, 12% Other, 6% Buddhist, and a small amount of Jews, some fraction of 1%. The data comes from 2005, courtesy of the World Christian Database (Center for the Study of Global Christianity at the Gordon-Conwell Theological Seminary) and there is nothing about number or methods. It doesn't really matter -- Americans are mostly interested in the Muslim figures in studies like this. The Nonbelievers figure is probably skewed by Communist countries, which all rank very high in Nonbelievers, with the exception of Laos -- and frankly I'm very skeptical as to what their data is on Laos anyway. This is all about Market Share.