Of the world's 7 billion people, 84 percent identify with some form of religion, according to a new comprehensive report on the global religious landscape released Tuesday.
Christians, with 2.2 billion followers, comprise the largest religious group of the world's population, followed by Muslims, with 1.6 billion. The 1.1 billion who say they are unaffiliated with any kind of religion are the third largest group.
The survey by the Pew Forum on Religion & Public Life shows the highest concentration of religious adherents in the Asia-Pacific area, where Christians and Jews are small minorities. The faith with the youngest population is Islam, with a median age of 23 years, while Judaism has the oldest median age of followers at 36 years.
The study assembled data from more than 2,500 national censuses, large-scale surveys and official population registers from more than 230 countries and territories to compile what scholars and experts say is the most comprehensive report on the world's religious makeup.
"People are beginning to realize that just about every area of human life is impacted by religion, and just knowing these basic facts can be very helpful," said Todd Johnson, director of the Center for the Study of Global Christianity at Gordon-Conwell Theological Seminary.
Youth and growth
The median age of each major religious group was one example of new and important data to come out of the Pew Forum's work, as it is an indicator of future growth or decline, said Johnson, who is also a co-editor of the World Religion Database, a source for the Pew study.
Only Muslims, at 23 years old, and Hindus, at 26 years old, have median ages that are younger than the median age of the world's overall population, which is 28 years. Christians have a median age of 30 years, followed by members of other religions (32), adherents of folk or traditional religions (33), the religiously unaffiliated (34) and Buddhists (34). Jews have the highest median age, at 36 years.
"There are a lot younger cohorts within Muslims internationally, and that bodes well for future growth," Johnson said. "It's not the only factor to determine growth, but it is certainly significant."
Conrad Hackett, a Pew Forum demographer, said the next step in the Pew research is to examine where religion is headed in the future, and age is among the leading indicators.
"All things being equal, median age is an important clue of which groups have an advantage or disadvantage going forward," Hackett said, noting fertility and mortality rates are also factors that predict growth.
One group that has seen recent growth in the United States is the religiously unaffiliated, or so-called "nones." According to the Pew study, the nones make up 16 percent of the world's population. The largest concentrations of the religiously unaffiliated are in communist countries such as North Korea, where 71 percent of the population does not identify with a faith, and China, where the figure is 52 percent.
Hackett said the unaffiliated group includes not just atheists and agnostics but also people who believe in God and consider themselves religious but don't identify with a specific faith tradition.
"There are a number of countries where Buddhism is part of the culture, but people don't consider it a religious identification — even though they have those rituals in their life," he said.
Johnson said a good example of that is Vietnam, where 30 percent of the population identifies as unaffiliated.
"But every person has a spirit house and is involved in daily religious practices in ways you won't believe," he said.
Concentration and disbursement
With nearly 60 percent of the world's population living in the Asia-Pacific region, it follows that the highest concentrations of Hindus, Buddhists, folk religionists, other religions, unaffiliated and Muslims live in the region, according to the Pew study.
Christianity is the most evenly dispersed religion, with the highest concentrations in Europe (26 percent of all Christians live there), followed by Latin America and the Caribbean (24 percent) and sub-Saharan Africa (24 percent). Another 13 percent of all Christians live in Asia and the Pacific, with 12 percent in North America.
Christians make up barely more than 1 percent of the population in the Middle East/North Africa region, where the faith originated, Pew noted.
Christians are youngest in sub-Saharan Africa, where their median age is 19 years. The oldest median age for Christians is in Europe at 42 years.
Those dispersion and median age factors tell the story of Christianity over the past century as its growth has shifted south through missionary efforts and the secularization of Europe.
"One hundred years ago, 80 percent of the Christians were in Europe and North America," Johnson said. "Today, it's only 40 percent."