Birth rates and geographic concentration will have the greatest influence on the growth of the world's religions in the next four decades, according to a new Pew Research Center report on the global religious landscape. Islam stands to be the fastest-growing faith, while the share of religiously unaffiliated will shrink.
By 2060, the Muslim population is predicted to grow by 70 percent because births will far surpass deaths within the faith community, Pew reported. If these calculations hold up, Muslims will comprise nearly one-third (31 percent) of the world's population and be within one percentage point of the previously dominant Christian community.
"Less than 20 years from now, the number of babies born to Muslims is expected to modestly exceed births to Christians," the new analysis noted.
Pew's report updates figures published in April 2015, extending demographic estimates by 10 years. It draws on more than 2,500 population registers from around the world, as well as the expertise of dozens of religious scholars.
This year's report also includes an overview of many Americans' flawed assumptions about the global religious landscape, which are influenced by U.S.-specific media coverage of religious "nones," immigration and adults who switch between faith groups.
Here are the key takeaways from the new report:
The biggest reason the Muslim population is surging as a share of the world's population is because of a high number of Muslim births.
From 2010 to 2015, 31 percent of babies born around the globe had Muslim moms, even though Muslims represented only 24 percent of the world's population in 2015.
The share of Christian births is also higher than the Christian share of the population, but only by two percentage points, which helps explain why the Muslim community will almost catch up to Christians by 2060.
An additional reason for this predicted shift is a high amount of deaths among Christians. "In recent years, Christians have had a disproportionately large share of the world's deaths (37 percent) — in large part because of the relatively advanced age of Christian populations in some places," like Europe, Pew reported.
Religious switching — or people's movement from one faith group to another or from a faith community to the community of religious "nones" — drives a large amount of research on and discussion of modern congregations. People wonder what causes young people to leave their childhood faith or how to evangelize more effectively.
However, these shifts play only a minor role in overall religious population changes. "The effects of religious switching are overshadowed by the impact of differences in fertility and mortality," Pew reported.
For example, the global Christian community lost 9 million members from 2010 to 2015 due to religious switching. But it gained more than 100 million devotees due to the difference in births and deaths.
The outsized influence of natural (or birth-driven) increases explains why religious "nones" will decrease as a share of the global population over the next four decades, in spite of gaining many people through religious switching.
"While religiously unaffiliated people currently make up 16 percent of the global population, only an estimated 10 percent of the world's newborns between 2010 and 2015 were born to religiously unaffiliated mothers," researchers noted.
Geographic concentration is another factor influencing religious population growth. "Religions with many adherents in developing countries — where birth rates are high and infant mortality rates generally have been falling — are likely to grow quickly," Pew reported.
Over the next 40 years, Christianity and Islam will benefit from having a growing share of their global populations in sub-Saharan Africa. Pew estimates that 42 percent of the world's Christians will live there in 2060, compared with 26 percent in 2015.
Religious "nones," however, will mostly increase their presence in more developed areas of the world, including North America, where there are aging populations and low fertility rates.
Overall, Pew's report runs counter to many Americans' expectations about global faith. For example, 62 percent of U.S. adults predict that religious "nones" will increase as a share of the world's population in the next few decades.
Just over half of Americans (52 percent) correctly believe that Christians represent the world's largest religious group, while 25 percent say it's Muslims and 15 percent say the religiously unaffiliated.
People's assumptions about the global religious landscape are influenced by a variety of factors, including their age, religion and political affiliation. Republicans are more likely than Democrats to predict that Muslims will become the largest faith group, while Democrats are more likely than their conservative counterparts to expect a rise in the religiously unaffiliated.
This latter assumption will at least be true within the U.S. context, according to the new analysis. "By 2060, 9 percent of the global unaffiliated population will live in the United States alone," up from around 5 percent in 2015, Pew reported.