religions of the world

The world's 20 largest religions and their number of believers are:

1. Christianity (2.1 billion)
2. Islam (1.3 billion)
3. Nonreligious (Secular/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 and Diasporic (100 million)
9. Sikhism (23 million)
10. Juche (19 million)
11. Spiritism (15 million)
12. Judaism (14 million)
13. Bahai (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 (800,000)

Taken from The Register article on religions. Some more interesting facts in there.