Pew - The Global Religious Landscape
A Report on the Size and Distribution of the World’s Major Religious Groups as of 2010
"Worldwide, more than eight-in-ten people identify with a religious group. A comprehensive demographic study of more than 230 countries and territories conducted by the Pew Research Center’s Forum on Religion & Public Life estimates that there are 5.8 billion religiously affiliated adults and children around the globe, representing 84% of the 2010 world population of 6.9 billion."