While the Buddhist population will remain the same, and the number of Hindus and Jews will increase, those unaffiliated with any religion—atheists, agnostics and others—will decline overall.

In 2010, the number of Christians (the world’s largest religious group) is estimated at 2.2 billion. Muslims come in second, at 1.6 billion adherents. While Christians are projected to rise to 2.92 billion, Muslims will grow by more than 1 billion, to 2.76 billion.

By 2050, the world’s overall population is projected to rise to 9.3 billion, a staggering increase of 35%. The number of Muslims, who are on average younger and have higher fertility rates will increase by 73%, double the growth rate of Christians. India will have the world’s largest Muslim population (even though Hindus will remain the majority). 40% of the world’s Christians in 2050 will live in sub-Saharan Africa, the report says.

“One of the main determinants of that future growth is where each group is geographically concentrated today. 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. Much of the worldwide growth of Islam and Christianity, for example, is expected to take place in sub-Saharan Africa.”

By contrast, the world’s agnostics and atheists live in areas with aging societies such as North America, Europe, China and Japan. The unaffiliated population will rise slightly, from 1.1 billion in 2010 to 1.2 billion in 2050, fueled in part by lapsed Christians. But their overall share of the global population will fall from 16% to 13%. In the US, people will increasingly shun religious affiliation, going from 16% of the country to 26%. Islam will surpass Judaism as the second largest religion in the country. In Europe, one in ten will be Muslim.