With 2.18 billion adherents, Christianity has become a truly global religion during the past century as rapid growth in developing nations offset declines in traditional strongholds, according to a report released Monday.

Billed as the most comprehensive and reliable study to date, the Pew Research Center’s “Global Christianity” reports on self-identified Christian populations based on more than 2,400 sources of information, especially census and survey data.

Findings illustrate major shifts since 1910, when two-thirds of the world’s Christians lived in Europe. Now one in four Christians lives in Europe. Most of the rest are distributed across the Americas (37 percent), sub-Saharan Africa (24 percent) and the Asia-Pacific region (13 percent).

“In two out of three countries in the world, the majority of the population identifies as Christian,” said Conrad Hackett, lead researcher on the “Global Christianity” report. “I had no idea about that. ... I was surprised.”

The report confirms Christianity’s standing as the world’s largest religion, with 32 percent of the global population. Islam is second with about 23 percent, according to a 2009 Pew report.

A close look at the details reveals a few ironies:

• Although Christianity traces its beginnings to the Middle East and North Africa, only 4 percent of residents in these regions claim the Christian faith today.

• The faith has grown exponentially in sub-Saharan Africa, from just 9 percent of the population in 1910 to 63 percent today. Nigeria, home to more than 80 million Christians, has more Protestants than Germany, where the Protestant Reformation began.

“As a result of historic missionary activity and indigenous Christian movements by Africans,” Hackett said, “there has been this change from about one in 10 [sub-Saharan Africans] identifying with Christianity in 1910 to about six in 10 doing so today.”

For its part, Europe is more religiously diverse than it was in 1910, when 94 percent was Christian. Still, Europe hasn’t abandoned its Christian heritage, according to the report. Today, 76 percent of Europeans self-identify as Christian.

“Many people may have the impression that a smaller percentage of Europe claims to be Christian” than is actually the case, Hackett said.

The report also sheds light on the difficult question of how many Chinese are Christians. Researchers have struggled to get reliable numbers since China’s policies on religion are thought to discourage Christians from self-identifying as such in official surveys.

Adjusting for such variables, Pew researchers believe Christianity has flourished despite a policy forbidding Christianity among Communist Party members. Researchers estimate the Christian community in China includes 5 percent of the population, or 67 million.