The total fertility rate in the US has fallen to 1.7. That means the average woman will have fewer than two children, not enough to replace the population. Does greater religiosity predict fertility, and if so, how strongly? I organized data on the total fertility rate (TFR)(World Population Review, 2021 data) and the percent of the country thinking religion is important (World Values Survey, various years) for 78 countries. The Spearman rank correlation is .61 (p < .001), which means that religiosity strongly predicts higher fertility. Here is a scatterplot:
Here is the scatterplot with country labels:
If we assume that a TRF of at least 2.1--replacement level--is best, then a country evidently needs close to half or more of the population believing that religion is important.