But perhaps the most significant finding is that Muslims may gradually overtake Christians as the world's largest religious group in the coming decades.

Projected Christian and Muslim Shares of the Global Population, 2010-2100

Pew Research Center

Consider the situation in Europe, where Christianity has historically been dominant. Now, it's the only region where the absolute number of Christians is expected to decline in the coming years. In 2010, 75 percent of Europe's population was Christian; by 2050, that percentage is expected to be closer to 65. In general, the continent's total population is expected to shrink.

Europe's population of Muslims, however, is expected to get much larger, growing by 28 million people over four decades. Muslims will still be a minority in the region, accounting for roughly 10 percent of Europe's population by 2050. But given that Islam's growing presence has already contributed to significant social and political conflict across the continent, this change could be consequential.

In Europe and beyond, age, fertility rates, and migration are the most important factors in projected population changes, but religious conversion also plays a minor role in the results. Despite Christianity's tradition of evangelism, the faith is expected to lose a net total of about 66 million people around the world due to conversions, accounting for both those who convert into the faith and those who convert out. A significant portion of those converts will likely become unaffiliated, a group that's expected to grow by a net total of roughly 61 million purely due to people leaving their religions (as opposed to via higher birth rates, etc.).

As the graph below shows, the majority of those who leave Christianity will be men; they're expected to account for roughly 60 percent of those who choose to leave the religion. Hackett said this is partially explained by the fact that women tend to have higher levels of religious commitment than men. It's also worth noting that this gender difference in patterns of conversion holds for Christianity but not Islam: In certain Muslim countries, Hackett said, there's little religious switching in general, partly because of the serious social and legal consequences people might face for a change of faith.

Projected Changes in the World's Religious Populations Due to Conversion, 2010-2050

Pew Research Center

The data seems to undermine the theory that the world is on an inevitable, historical march toward secularization. The number of people who don't claim any particular religion is expected to rise modestly in the next few decades, but the world is going to grow much faster than the religiously unaffiliated population will. Most non-religious people live in China, Japan, and the United States, with significant numbers in Europe as well—all places with relatively old populations and low birth rates. Forty years from now, the non-religious portion of the world's population will still only be about 13 percent, which is slightly less than what it was in 2010.