Having kids is one of those things. For example, in the US a woman who is considered religious based on frequency of services attendance has on average almost one child more than a woman considered nonreligious. ([0] presents one such chart for the United States.)
Except for Buddhism. For some reason, Buddhist women have lower fertility than even atheists [1] I find this surprising and don't really understand the reason.
So anyways, Korea seems to be doubly affected - it has one of the world's highest rates of non religiosity [2], and out of those few who are religious, most are Buddhist.
It seems like if you want to encourage population growth in your country, you want to encourage adherence to an Abrahamic religion or Hinduism.
[0] https://ifstudies.org/blog/americas-growing-religious-secula...
[1] https://www.google.com/amp/s/www.worldatlas.com/amp/feature/...
[2] https://en.m.wikipedia.org/wiki/List_of_countries_by_irrelig....