Take note, this is how you make a source---
http://en.wikipedia.org/wiki/Religion <----- source. A link.
"Present day religious adherence and trends
Christianity is the religion with the largest number of professed adherents, followed by Islam and Hinduism. These statistics show the number of professed adherents of the major world religions. In addition, approximately one billion people do not profess any belief in a religion. These figures are necessarily approximate."
Christianity: 1.9 billion
Islam: 1.3 billion
Hinduism: 1 billion
Buddhism: 400 million
Chinese traditional religion: 394 million
Primal-Indigenous: 300 million
African Traditional & Diasporic: 100 million
Sikhism: 23 million
Spiritism: 15 million
Judaism: 14 million
Bahá'í: 7 million
Jainism: 4.2 million
Shinto: 4 million
Cao Dai: 4 million
Zoroastrianism: 2.6 million
Sant Mat / Surat Shabd Yoga : 2 million
Tenrikyo: 2 million
Unification Movement: 1.5 million
Neo-Paganism: 1 million
Unitarian-Universalism: 800 thousand
Rastafari movement: 600 thousand