Christianity has long been the world’s largest religion by far, but the population of Muslims is growing so fast that they will match Christians by the year 2070 and outnumber them by the end of the century, according to a report released Thursday that projects the global religious future.

The report, from the Pew Research Center, projects a vibrantly religious planet, not the withering away of religion predicted by some futurists. The reason is not that religious groups will win significantly more converts, but simply that religious adherents are younger and have more children than secular people.

Those demographic factors will drive the growth of Islam because Muslims are the youngest and have the highest fertility rates of any religious group, the report says.

In the United States, the spread of secularism will probably continue: Those who claim no religion will make up about a quarter of the population by 2050 — an increase from 16 percent in 2010. Christianity will have the biggest losses, with its share of the American population declining to 66 percent in 2050 from 78 percent in 2010, according to the projections in the report.