Christians are the world's biggest religious group, numbering some 2.2 billion people, according to a study released on Tuesday by the Pew Forum on Religion and Public Life.
Pew assembled data on the size and geographic distribution of eight major religious groups, including non-believers.
It found that Christians make up about 32 percent of the world's population, followed by Muslims, the second largest group, with 1.6 billion adherents.
Hindus were the third largest group, numbering about one billion (15 percent), followed by Buddhists, at 500 million (seven percent) and Jews, who number 14 million (0.2 percent.)
The worldwide demographic study of more than 230 countries and territories found that more than eight people in 10 - about 5.8 billion people - identify with a religious group.
More than 400 million people (six percent) practice various folk traditions, including African traditional, aboriginal or folk religions, the global survey found.
The Pew Forum said that the study, in which religious affiliation was based on self-identification, did not attempt to measure the degree to which practitioners observe their faiths.