No one religion holds the majority of the world's people.
The largest religion is Christianity, at 33%; next Muslim at 21%; Hindus at 13%; Buddhists at 6%; non-religious at 14%, and other at 12%. (See http://en.wikipedia.org/wiki/World_religions)
As for the reason why, some would say, of course, that God has a hand in it. I won't speak to that. I'll look at social and political factors.
The two largest religions, Christianity and Islam, proselytize and seek to convert, and in the recent historical past, have violently coerced people to convert. The largest, Christianity, is the dominant religion of many recent imperial powers, including Britain and Spain. Both spread Christianity worldwide.
In contrast, Hinduism is large because it is the indigenous religion of a very large subcontinent, India. Buddhism is large because it spread peacefully throughout India, China, East Asia, Southeast Asia and Tibet. The non-religious group is large due to both modern secularism and the communist (officially non-religious) People's Republic of China.