Islam is expected to grow faster than any other religion in the world over the next 30 to 40 years, according to a Pew study.

The world is becoming more religious, as the number of agnostics and others who don't affiliate with a certain religion shrinks as a percentage of the global population.

By 2050, just 13 percent of people in the world will say they are unaffiliated, compared with 16 percent who said the same in 2010, according to a new Pew Research Center survey.

The United States is an exception, where more Americans are expected to flee organized religion. [8 Ways Religion Impacts Your Life]

Religious risers

Numbers for all of the world's major religions, except Buddhism, are expected to rise as the population does the same.

Islam will grow faster than any other major religion, and at a higher rate than the world population balloons, the survey found. In fact, Muslims are projected to increase by 73 percent between 2010 and 2050. If current trends hold, Christianity will also grow, albeit at a slower rate, increasing by 35 percent by 2050. That is about the same rate as the world's population overall is expected to grow by 2050.

If those numbers pan out, there will be nearly equal numbers of Muslims (2.8 billion) and Christians (2.9 billion) in the world by 2050, for the first time in history. Increases in a slew of other religions are also forecast: Hindus are projected to rise by 34 percent, from just over 1 billion in 2010 to 1.4 billion in 2050; Jews are expected to grow from just under 14 million in 2010 to 16.1 million by 2050

Also by 2050, some 450 million people in the world will be affiliated with various folk religions, such as African traditional religions, Chinese folk religions, Native American religions and Australian aboriginal religions, the survey projected. That represents an increase of 11 percent relative to 2010 numbers.

Nonbelievers and switchers

People who don't believe in any gods as well as agnostics and those not associated with a particular religion will become a smaller slice of the world's population. Though this unaffiliated group will increase in numbers from 1.1 billion to 1.2 billion, it will account for a lower percentage of the population (16 versus 13 percent) in 2050.

That won't be the case in the United States, however, where agnostics, atheists and other unaffiliated individuals will increase from 16 percent to 26 percent of the population by 2050, Pew found. Christians in the United States are predicted to decline from 2010's 78 percent to 66 percent by the middle of the 21st century. By that same date, Muslims (2.1 percent of population) are expected to outnumber Jews (1.4 percent) in the United States.

These shifts in the world's religions are the result of several factors, including differences in fertility rates, the size of the youth population and people switching faiths, Pew said. (Younger populations have more people with prime childbearing years ahead.)

For instance, a good chunk of the growth in Christianity and Islam is expected to happen in sub-Saharan Africa, where birth rates are high. Fertility rates varied by religion, according to Pew, with Muslims having the highest fertility rate, of 3.1 children per woman; Christians coming in second, with 2.7 kids per woman; Hindus and Jews with average fertility rates of 2.4 and 2.3, respectively; and Buddhists having one of the lowest fertility rates, at 1.6.

"Today's religiously unaffiliated population, by contrast, is heavily concentrated in places with low fertility and aging populations, such as Europe, North America, China and Japan," according to a statement by Pew.

Changing one's religious affiliation will also affect these numbers, with Christianity forecast to have the biggest exodus, losing 106 million believers by 2050 while gaining just 40 million. Most of those leaving Christian religions will choose not to be affiliated with a particular religion, Pew predicts. About 3 million people are expected to become Muslims, while 3 million Buddhists and 300,000 Jews are forecast to switch out of those religions, the report found.

The Pew results are based on censuses and surveys for 175 countries, with statistics for religions in the remaining 59 countries coming from the World Religion Database and other sources, according to Pew. Researchers at the Age and Cohort Change Project of the International Institute for Applied Systems Analysis made the projection calculations using a modified version of the so-called cohort-component method, which is the standard demographic method for population projections. Pew has a detailed description of their methodology online.

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