We typically think the primary work of spreading and growing a religious faith is through sharing that belief with unbelievers. Missionary work. Evangelization. Proselytizing.

But a new study from the folks at the Pew Forum tells us the story is quite different for the coming decades. Babies are the deal, and the big story concerns whether Christianity or Islam will grow the fastest as a result.

Christianity is and has long been the world’s largest religious faith. Christians are also the most fertile of all religions overall. They like babies. Thirty-one percent of the world’s 7.3 billion people currently identify as Christians. Muslims represent nearly a quarter. All other religions are significantly lower. While Christianity is expected to continue to grow at a small rate, Islam is projected to make greater gains, coming within a decimal point by 2060. They are expected to surpass Christianity in baby-making work by 2035.

While not all babies grow to adopt their parents’ faith, most do. It is a sociological truism that children tend to continue the religious traditions and overall lifestyles in which they were raised. That Christians and Muslims are so fertile in a world where birth rates are nose-diving among most peoples and nations is no small part of their growth and strength.

The Muslim Population Is Younger and More Fertile

In the last five years, 33 percent of the world’s babies were born to Christians and 31 percent were born to Muslims. By 2060, this fertility score will be 35 to 36 percent in favor of the Muslims. It will decline from 16 to 13 percent for those with no faith.

This is all driven by a current fertility rate for Muslims of 2.9 children per women over Christianity’s 2.6. Religiously unaffiliated women have a birth rate of 1.6 children, below replacement level of 2.1. This number is unlikely to increase. As a result, by 2060, 31 percent of the world’s population will be Muslim, compared to 32 percent who will identify as Christian. All other religions, as well as the infamous “nones” or unaffiliated, are expected to shrink as a percentage of the population, with the exception of Jews, who will remain stable at 0.2 percent.

Of course, the other side of the coin of births is deaths, and Muslims have a slight advantage here. The current and coming cohort of Muslims tends to be a bit younger than Christians. This will serve to turbo-charge their slightly higher fertility rate in terms of overall growth.

Think of it this way. If you’re in the diaper business, you’ll find more business among Muslims. Their younger overall demographic means having more babies. If you’re in the Rascal mobility scooter or Jitterbug cell phone businesses, Christians will be your more lucrative market in the coming decades. Younger people-groups have more babies. Older people die sooner. These things matter.

The Geographic Centers of Both Religions Shift to Africa

The futures of Christianity and Islam have a very strong geographic dynamic as well. The burgeoning market for Bible and Koran sales will remain robust, but it will be located in sub-Saharan Africa. Into 2060, the share of Christians in the world making their home there will increase from 26 to 42 percent! It will shrink in Europe and North America.

For Muslims, 27 percent of their global population will be living in sub-Saharan Africa, up from 16 percent today. Their greatest decline is expected in the Asia-Pacific part of the world, 61 percent down to 50 percent. Both Christianity and Islam will remain steady in the Middle East and North Africa.

The bad news for Christianity is not just more deaths and a slightly slowing birth rate, but people actually leaving Christianity. Pew projects it will lose 8 million adherents while the unaffiliated will grow by just less than that number by 2020. Nearly all of this loss in Christianity is seen and will continue among mainline churches that are busy jettisoning many of the fundamentals of the faith. Their people can’t get to the doors fast enough.

Muslims will gain 420,000 adherents through new converts. So, while Christianity will remain the world’s largest religion by a small margin over Islam, it will lose a great deal of people in the coming decades while Islam and those with no particular faith will increase in real numbers.

But projections are not destiny. A cool thing about the future is it hasn’t happened yet, and the future of a faith rises and falls, not in projections, but in the real-world behaviors and fidelity of its adherents. Do they live their faith in a public and truthful way? And are they having babies while they’re doing so? These things matter and are deeply consequential.