What are the most common religions?

Answer 1

Christianity (if one includes all the protestant divisions with Catholicism and the orthodox religions) is about one third of the world's population. Islam has more than a billion adherents, and is over 20%. Hinduism and Buddhism are both quite large, roughly 14% and 6% of the world's population.

Answer 2

These numbers aren't exact, but they are pretty close.

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