On Jan. 19 I did a podcast on the Remaining Task for MissioNexus. One of the questions posed then was the following, which I answered online but I am also answering here for the benefit of all:
In recent years, I've heard that the evangelical church is growing more rapidly than Islam. Your statements seem to discredit that idea. Please elaborate.
The raw numbers are as follows:
In the context of the talk I note that Islam is the fastest growing religion in the world, but I am generally comparing it to the "top line numbers" - e.g. vs. Christianity as a whole, or Hinduism, or Buddhism, etc.
Evangelicals are definitely growing faster than Islam. There are two reasons why this is so.
First, evangelicals are a smaller group, and smaller groups have faster growth rates. For example, let's say you have a family of 2 (a husband and wife). They have twins. In one day, the family has grown 100% - it has doubled - simply by adding 2 more people. If that same family then, a few years later, had another set of twins, they would only have grown 50% - because 2 vs. 4 (the family size of husband, wife, and 1 set of twins) is just 50%. (If a family of 2 had quadruplets, they would have grown by 200%!).
Second, evangelicals are growing faster than many strands of Christianity because they not only share the same birth rates (as a rule) but also often have higher conversion rates. Global religious dynamics in any given place are births + immigrants + converts - deaths - emigrants - defectors. Islam doesn't have as high a conversion rate as evangelical Christianity does.
You will find this in other subsets of Christianity, too. The Protestant tradition is growing at 1.64%, the Catholic tradition at 1.08%. The Independent tradition as a whole is growing at 2.21% - faster than Islam - and Asian Independents in particular are growing at 2.94%!.
But speed rates don't tell you everything. Just because Asian Independents have this remarkable speed doesn't mean they're going to overtake Islam or the rest of the world. Asian Independents number 154 million; growing at 2.94% p.a. means this year they will add 4.5 million new members. Islam, by contrast, has 1.7 billion, and at 1.93% per year will add 32.8 million. It is a third as fast (rate-wise) but will add nearly 10x as many people.
So this is the caveat: we must watch the fantastic headlines. Yes, "Islam is growing faster than Christianity," but that doesn't mean it will overtake Christianity in size. The turtle might be going twice as fast as the rabbit, but if the turtle is 4x further away from the goal, it won't reach it in time. And yes, "Evangelicals are growing faster than Islam," but they won't overtake Islam in the future (for exactly the same reason).
We need more work, in more places, at faster rates. Which is why I ascribe to mobilization and movements.