Jim Hollander / EPA

Franciscan nuns and Nigerian Christians pray inside St. Catherine's Church, adjacent to the Church of the Nativity, traditionally accepted as the birthplace of Jesus Christ, in the West Bank town of Bethlehem on Monday.

Roughly one in six people around the world has no religious affiliation, a new study by the Pew Research Center’s Forum on Religion & Public Life found, making the unaffiliated the third-largest religious group worldwide, behind Christians and Muslims, and about equal in size to the world’s Catholic population.

The religiously unaffiliated population includes atheists, agnostics and people who do not identify with any particular religion in surveys, the study issued Tuesday reads. Many of the religiously unaffiliated, however, hold religious or spiritual beliefs, the study emphasized.

"For example, belief in God or a higher power is shared by 7 percent of Chinese unaffiliated adults, 30 percent of French unaffiliated adults and 68 percent of unaffiliated U.S. adults," it read.

Making up 16.3 percent of the world population, this group comprises a majority of the population in six countries. China's number of religiously unaffiliated is the largest, with a 62 percent share.

The Pew Forum's study is based on self-identification.

Titled "The Global Religious Landscape," the study analyzed data available as of early 2012 from more than 2,500 national censuses and large-scale surveys, and found that Christians are the world's biggest religious group, with 2.2 billion people or 32 percent of the world’s population. The largest share of all Christians live in the United States, followed by Brazil and Mexico.

About half of all Christians are Catholic, while an estimated 37 percent of Christians are Protestant, the study shows. Greek and Russian Orthodox Christians make up 12 percent of Christians.

With 23 percent of the world's population, Muslims represent the second-largest religious group and are a majority in 49 countries, including 19 of the 20 countries in the Middle East and North Africa.

Hindus make up 15 percent of the population, while the nearly 500 million Buddhists add up to 7 percent.

The study also found that the median age of Muslims (23 years) and Hindus (26) is younger than the median age of the world’s overall population (28), and more than 12 years younger than the median age of Jews, which is 36 years old.

"Muslims are going to grow as a share of the world's population, and an important part of that is this young age structure," Pew Forum demographer Conrad Hackett told Reuters.

Judaism has the weakest growth prospects in comparison.

There are about 15 million Jews in the world, or about 0.2 percent of the global population, and about 44 percent of them live in North America, while about 41 percent live mostly in Israel.

The Pew Forum study also shows that an estimated 405 million people practice various folk or traditional religions, including African traditional religions, Chinese folk religions, Native American religions and Australian aboriginal religions. More than 70 percent of the world’s folk religion practitioners live in China.

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