The number of Americans who do not identify with any religion has been growing steadily in recent years. But in coming decades, religiously unaffiliated people are expected to make up a declining share of the world’s population, according to a recent study by the Pew Research Center.

Other than Buddhists, all of the world’s major religious groups are poised for at least some growth in the coming decades.

By 2050, the number of Muslims around the world will nearly equal the number of Christians. Pew projections suggest that Muslims will make up nearly one-third of the world’s population of about 9 billion people.

“The culture of the West is going to become increasingly nonreligious at the same time the culture in the Global South persists in being religious,” said David Voas of the Institute for Social and Economic Research at the University of Essex. “Repercussions will be global.”

The world could see a growing divide between the religious and nonreligious, which could have implications for global economic development, said Jack Goldstone, a professor of public policy at George Mason University.

Sociologists jumped the gun when they said the growth of modernization would bring a growth of secularization and unbelief, Goldstone said. “That is not what we’re seeing,” he said. “People want and need religion.”

In six countries with large unaffiliated populations (Japan, the United States, Vietnam, Germany, France and Britain), the number of those with no religion is expected to increase in the coming decades. But those countries also have populations that are shrinking as a share of the world’s population.

Pew projects Muslims to increase by 73 percent in population size from 2010 to 2050. If trends continue, Muslims, who have the highest fertility rate among major religious groups, will outnumber Christians after 2070.

China, which has the most people in the world, is difficult to measure. Because reliable figures on religious switching in China are not available, Pew’s projections do not contain any forecast for conversions in the world’s most populous country.

Pew demographer Conrad Hackett said an estimated 5 percent of Chinese are Christians. Purdue University sociologist Fenggang Yang estimates that the annual growth rate of Protestant Christians will continue at 10 percent, suggesting that China could become the largest Protestant country by 2021 and the largest Christian country by 2025. If growth continues, two-thirds of China could be Christian in 2050.

“That would be remarkable if that were to come to pass,” Hackett said at a gathering at Pew on Thursday. “China is probably the biggest wildcard in our report.”

Muslims in the United States are expected to surpass Jews as the second-largest religious group. In 2050, Pew expects 2.1 percent of the American population to be Muslim — more than double a 2010 estimate of 0.9 percent and higher than the 1.4 percent who will be Jewish.

There has been a significant shift in where Christians are located globally. In 1910, two out of three Christians lived in Europe. By 2050, only 16 percent of the world’s Christians will be in Europe.

Christianity is expected to lose a large number of followers, but it will still be the dominant tradition halfway through the century, thanks to projected growth in Africa and other areas of the global south. Four out of every 10 Christians in the world will live in sub-Saharan Africa.

The study, which Pew describes as the first of its kind, bases its projections on the age of populations, fertility and mortality rates, and it includes migration and conversion patterns. So even apart from religious conversions, powerful demographic trends such as fertility and migration drive the size of religious groups around the world.

India is expected to hold the world’s largest population, retaining its Hindu majority while also including the largest population of Muslims, surpassing Indonesia. The percentage of people in Europe who are Muslim is expected to climb to 10.2 percent, up from 5.9 percent.

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