Comment by Adverblessly

I can't speak to other countries, but here in Israel at least, religion is highly correlated with number of children.

Focusing on jewish women, fertility rates for different levels of religion in 2021-2023:

  Ultra-orthodox                6.48
  Religious                     3.74
  Traditional-religious         2.81
  Traditional, not so religious 2.20
  Not religious, secular        1.96

So naturally over time the religious portion of the population grows.