"The main reason Muslims are growing not only in number but in share worldwide is because of where they live," Alan Cooperman, Pew's director of religion research, tells NPR's Tom Gjelten. "Muslim populations are concentrated in some of the fastest-growing parts of the world."

The finding is part of the center's report on the future of the world's religions. You can see the full report at the Pew site, which has also published an interactive tool to help readers drill down by geography and religion.

"As of 2010, Christianity was by far the world's largest religion, with an estimated 2.2 billion adherents, nearly a third (31 percent) of all 6.9 billion people on Earth," the Pew report says. "Islam was second, with 1.6 billion adherents, or 23 percent of the global population."

Those numbers are predicted to shift in the coming decades, as the world's population rises to 9.3 billion by the middle of this century. In that time, Pew projects, Islam will grow by 73 percent while Christianity will grow by 35 percent — resulting in 2.8 billion Muslims and 2.9 billion Christians worldwide.