] Christianity: 2 billion
] Islam: 1.3 billion
] Hinduism: 900 million
] Secular/Nonreligious/Agnostic/Atheist: 850 million
] Buddhism: 360 million
] Chinese traditional religion: 225 million
] primal-indigenous: 150 million
] African Traditional & Diasporic: 95 million
] Sikhism: 23 million
] Juche: 19 million
] Spiritism: 14 million
] Judaism: 14 million
] Baha'i: 6 million
] Jainism: 4 million
] Shinto: 4 million
When reading these statistics, the number I found most surprising was the relative tininess of the world's Jewish population (14 million) in comparison with other major world religions (1 billion-plus). For some reason I'd thought it was a larger religion, perhaps because I know so many people of that faith. It made more sense as I dug deeper though, since approximately half of the world's Jews (about 6 million) live in the U.S., half in Israel, and the next largest percentage is listed simply as "Europe".
It puts some of the conflicts in the Middle East into a different perspective for me though. That the world's first two largest religions, Christianity and Islam, should seem to be so distracted -- even obsessed -- with a religion that is 1% of their size.
I also found interesting the listed quantity of non-religious people in the world, 850 million. Though it's worth keeping in mind that the number is probably somewhat skewed by the state-enforced atheism of China's 1+ billion inhabitants. With China included in the numbers, the world's population breaks down to about 80-85% religious, 15-20% atheist. Removing China from the mix though, a more common percentage around the world seems to be 97% religious, 3% non-religious. Perhaps 90-10, depending how agnostics are categorized.
Major Religions Ranked by Size