In the United States, the spread of secularism will probably continue: Those who claim no religion will make up about a quarter of the population by 2050 -- an increase from 16 percent in 2010. Christianity will have the biggest losses, with its share of the American population declining to 66 percent in 2050 from 78 percent in 2010, according to the projections in the report.

The number of Muslims in the United States will surpass that of Jews (at least those who claim "Jewish" as their religious identity) by 2035, but both groups will remain tiny portions of the American religious landscape, said Conrad Hackett, the lead researcher and demographer for the Pew report.

In Europe, the percentage of Muslims will rise to about 10 percent from about 6 percent of the population in the four decades from 2010 to 2050. That is a significant minority, but the picture is hardly that of the Muslim-dominated "Eurabia" that some nativist and anti-immigrant groups in Europe have warned of.

"We just don't see that happening," Hackett said.

He said that within one or two generations of arriving in Europe, Muslim immigrants -- like immigrants in general -- tended to leave behind the high fertility rates of their home countries and had smaller families, much like other Europeans.

"In the next generation or two, those immigrant families increasingly look like other families," Hackett said.

The report, six years in the making, projects the change in religious affiliation from 2010 to 2050 by region and by country. Its projections draw on mortality and fertility rates, age assessments, and patterns of migration and religious switching. The data came from more than 2,500 censuses, surveys and population registers from all over the world.

"Wars, famines, religious revival and all kinds of things could change the picture," Hackett said.

Demographers, religious leaders and policymakers are likely to pore over the results, said David Voas, a professor of population studies at the University of Essex who looks at religious trends. He has read the report and has provided comment for its authors.

"This is the most authoritative analysis we've had at the global level of the future of the religious population," Voas said. "It will provide a good foundation for debates and give people a clearer picture of what's likely to happen."

The biggest uncertainty is what will happen in China, the most populous country, whose 1.3 billion people have a tremendous effect on global trends. So many churches there are underground and not approved by the government that reliable figures on religious affiliation and switching are not available, the researchers said.

They estimated that in 2010, about 5 percent of China's population was Christian, 18 percent was Buddhist, 22 percent practiced folk religions and more than 50 percent had no religious affiliation. If notable percentages of China's unaffiliated converted to Christianity -- as some religion scholars predict -- the global religious projections could change significantly.

"It's undoubtedly the case that the current government in China has been keeping the lid on religious growth," Voas said. "If at some point in the next few decades there are political changes in China that open up the country to more religious freedom, I think it's not implausible that there would be a significant growth in Christianity, in particular, but perhaps other religious movements as well."

The shift in Christianity -- from a faith identified as belonging to white Westerners to a faith belonging to Africans, Asians and Latin Americans in the global south -- will intensify in the next 35 years, the report says.

Now, about 25 percent of the world's Christians live in sub-Saharan Africa, a trend already affecting debates over leadership and direction in churches such as the Roman Catholic Church, the Anglican Communion and the United Methodist Church.

By 2050, four in 10 Christians in the world are expected to live in sub-Saharan Africa. Even though the Muslim share of the population there will grow slightly faster than the Christian share, Christianity will still be the largest faith in the region and an increasingly powerful presence globally. Already, in a historical reverse, African churches are sending missionaries and priests to the United States and Europe, where faith appears to be in retreat.

"I think it can only be a matter of time before we have an African pope, because it will just seem reasonable," Voas said.