So which is it? I see factors going back and forth, and I will define "Christianity" broadly as any of these groups that claim to be Christian for the sake of this discussion (although I personally hold it to be a more narrow definition of only being the Church I believe is exclusively the one Christian Church).
On the one hand, we have groups like the Amish or Mormons who reproduce more than the general nonreligious population. While I see lots of people defecting from religion, I also see a lot of people doubling down on trying to take religion seriously. Those who have defected from Christianity seem to still seek out some kind of substitute religion, whether it is in science, New Age, Buddhism, or what have you.
So what do you think? Is "Christianity" growing or declining?