This is the conclusion of Philip Longman, a researcher at the New America Foundation. He wrote The Empty Cradle, about fertility decline across the world. In an excellent interview with Longman in Books and Culture, Brad Wilcox asked Longman what difference religious faith made in the future demography of the world. Longman argued that the people who do have larger families – enough kids to replace themselves, and then some – where much more likely to be religious.
One of Longman's most remarkable numbers is that the fertility gap between believers and nonbelievers in Europe is up to 15 or 20% now. Even in places where non-religious people are dominant in society today, they will have a harder time reproducing their power in the future because they are not reproducing in the most basic sense. Among the liberal educated elite, more than a third have no children or only one.
Fertility is indeed rising a little in Europe, and several European governments are pushing pronatalist policies for the first time in generations. Longman is skeptical that government programs can really nudge the birthrate much. He thinks the short-term financial advantages of no or few children will keep the birthrate down, especially for the educated. It is getting to the point where it takes a strong pronatalist family life – not public policy – to actually get people to bear the costs of kids. Religious people are most likely to value having a bunch of kids, despite the cost. And thus they inherit society, by degrees, and by default.