While Christians remained the world’s largest religious group at the end of the decade that ended in 2020, Christianity’s growth did not keep up with global population increase. But Islam — the world’s fastest-growing major religion — increased its share of the world population, as did the religiously unaffiliated, the Pew Research Center found in a report released Monday.

Even as the overall number of Christians — counted as one group, across denominations — continued to climb to 2.3 billion, the religion’s share of the world’s population decreased by 1.8 percentage points to 28.8 percent, a falloff driven in large part by disaffiliation. The Muslim population, on the other hand, increased by 1.8 percentage points to 25.6 percent, according to the report, which examined changes in religious demographics through an analysis of more than 2,700 censuses and surveys.

“It’s just striking that there was such dramatic change in a 10-year period,” said Pew’s Conrad Hackett, the lead author of the report. “During this time, the Muslim and Christian populations grew closer in size. Muslims grew faster than any other major religion.”

The report attributed the growth in Islam to a younger Muslim population — with an average age of about 24, as opposed to a global average age among non-Muslims of about 33 as of 2020 — along with higher fertility rates in some areas and lower rates of disaffiliation as compared with other religions, including Christianity.

“Among young adults, for every person around the world who becomes Christian, there are three people who are raised Christian who leave,” Hackett said.

The largest share of Christians — about 31 percent — can be found in sub-Saharan Africa, according to the study. Previously, Europe was home to the largest number of Christians in the world.

“And that’s the result of high fertility, youthfulness and rapid growth in general of sub-Saharan Africa,” Hackett said, “as well as the aging, lower fertility and disaffiliation going on in Europe.”

Religion switching — seen in high numbers among Christians — contributed to the global rise of unaffiliated people, the report found. Nearly a quarter of the world’s population did not identify with a religion in 2020 (24.2 percent), as opposed to 23.3 percent in 2010.

Researchers previously predicted the unaffiliated population would shrink because of older age and fewer people having children, Hackett said. However, people leaving religion, and especially departing Christianity, has led to the increase of the population, Hackett said.

The world’s biggest unaffiliated population is estimated to be in China: 1.3 billion people, out of 1.4 billion, followed by the United States, with 101 million disaffiliated out of 331.5 million, and Japan, with 73 million out of 126.3 million.

In China, it is common for people to have religious beliefs, but only 10 percent of the population formally identifies with a particular denomination or religion, the Pew Research Center reported in 2023. In a similar way, the new report measures the self-described affiliations of people around the world and may not capture the nuances and fullness of complex and evolving identities. Many people hold religious or spiritual beliefs or attend worship services but eschew formal labels.

Another group that underwent significant a loss in population were Buddhists, the only religion that had fewer members in 2020 (324 million) than in 2010 (343 million); this was due to disaffiliation and a low birth rate. Those who identified as Hindu and Jewish maintained rates steady with the world’s population, the report found.

“Sometimes we hear rumors of religious revival, and it’s certainly possible that in particular places religion could grow,” Hackett said. “But in this careful 10-year study that we’ve done, the broad trend is that in many places people are moving away from religion.”

Based on patterns of religion switching and differences in age and fertility, Hackett estimated that the “movement towards convergence” between Christians and Muslims will continue, with Islam set to grow to become the world’s largest religion in the years ahead, unless trend lines shift.

“The next step of our ongoing work in this project will be to do some demographic population projections to provide new estimates of sort of exactly when they might converge,” Hackett said. (The Washington post)