CIA estimates 33% of world is Christian

According to the CIA Christianity is the world's largest religion.

From CNN.com:

CARL AZUZ, CNN STUDENT NEWS REPORTER: [...] The Central Intelligence Agency estimates that the world's major religions are represented like this: 33 percent of humans are Christian. Muslims make up about 20 percent of the world's population... and Hindus account for just over 13 percent. Buddhists make up almost 6 percent... And a little more than 14 percent of the world's people are not religious.

Buddhist CIA Demographics Hindu Islam
Christian Christianity