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In terms of numbers, Christianity is the world's most popular religion, with approximately 2.1 billion followers. This includes just over a billion Catholics, about 350 million Protestants, and the Orthodox and Anglican faiths. The religion statistics resource Adherents.com hosts a fascinating pie graph that breaks down the world's major religions by population.

Islam is the world's second-largest religion, with roughly 1.3 billion followers -- including 940 million Sunnis and 170 million Shiites, as well as those belonging to other Muslim faiths like Sufism. Depending on your resources, the third-largest group is either Hindus or atheists. Both hover just under the billion mark. Traditional Chinese religion (which is related to, but distinct from, Buddhism) takes fourth place with 400 million, and Buddhism trails close behind.