Sizes shown are approximate estimates, and are here mainly for the purpose of ordering the groups, not providing a definitive number. This list is sociological/statistical in perspective

http://www.adherents.com/images/rel_pie.gif

1 Christianity: 2.1 billion
2 Islam: 1.5 billion
3 Secular/Nonreligious/Agnostic/Atheist: 1.1 billion
4 Hinduism: 900 million
5 Chinese traditional religion: 394 million
6 Buddhism: 376 million
7 primal-indigenous: 300 million
8 African Traditional & Diasporic: 100 million
9 Sikhism: 23 million
10 Juche: 19 million
11 Spiritism: 15 million
12 Judaism: 14 million
13 Baha’i: 7 million
14 Jainism: 4.2 million
15 Shinto: 4 million
16 Cao Dai: 4 million
17 Zoroastrianism: 2.6 million
18 Tenrikyo: 2 million
19 Neo-Paganism: 1 million
20 Unitarian-Universalism: 800 thousand
21 Rastafarianism: 600 thousand
22 Scientology: 500 thousand

Source  Adherents.com