It has been statistically proven that White Christians (mainly protestant denominations) are having children above the replacement rate, and are overall higher than the non-religious. Its also proven that the reason theyre not facing the population boom predicted by their higher fertility rates is due to the fact that a substantial number of their offspring end up becoming apostates who abandon the church, furthermore solidifying that Christianity and involvement in the church is directly responsible for higher fertility rates.
"Obviously, fertility rates are in decline for each of the four samples because each sample is getting precipitously older, which drives down fertility. With that in mind, the overall trend for each of the four groups is strikingly similar. The white birth rate, which was between 2.2 and 2.5 child per adult in 1972 has plummeted to 1.8 and 2.1 children per adult in 2016.There are some variations between the traditions, however. Note first that Catholics were never the tradition that had the highest birth rate, despite the Catholic Church’s position on birth control. As mentioned here previously, Catholics are not keen on following the Church’s position in this area and that bears out in this data. It’s clear that evangelical Protestants have the highest fertility rate. In fact, white evangelicals have children at a rate that is 15-20% higher than white Americans, in general. That should be a good thing for the future of white evangelicalism. The more children, the more likely members in the future. Despite this, as we have written about extensively, white evangelicals’ share of the population has not changed in the last decade (as shown below)."
So, if evangelicals are having a reasonable amount of children (above the 2 person population replacement rate) but their numbers overall are not increasing, what’s going on? Well, the answer could be multifaceted, obviously. But if I could hazard a guess it’s because white evangelicals have a leaky boat. That is, while young people start out in church, lots of them defect to other religions or no religion when they come of age. Hout and Fischer made this claim back in 2002; and Djupe and friends (2017) found that many Americans left religion after the political fallout of the 2016 elections.
(https://religioninpublic.blog/2018/01/29/the-graying-of-white-evangelicalism/#:~:text=In%20fact%2C%20white%20evangelicals%20have,fallout%20of%20the%202016%20elections.)
"Conservative Protestants who believe the Bible is the Word of God have higher fertility rates (around 1.8 children per woman) and less-negative conversion rates, meaning they only need to have around 2.4 children per woman. As a result, liberal denominations can expect a 48% decline in a generation, vs. just a 26% decline for conservative denominations"
On the other hand, nondenominational churches have experienced robust growth. Liberal nondenominational movements have roughly the fertility rate they need for stability (1.8 or 1.9), but conservative nondenominational movements have seen extraordinary growth: they only need to have about 0.8 children per woman to grow, yet in fact have around 1.9. This means they will more than double in size over the next generation, even without immigrants.
(https://ifstudies.org/blog/americas-growing-religious-secular-fertility-divide#:~:text=Liberal%20Protestant%20denominations%20appear%20to,decline%20in%20a%20generation%2C%20vs.)
