I recently (again) ran across the line, "Demography is destiny." Then, I encountered this link from 2016: "Which U.S. religious groups are oldest and youngest?" The two together illustrate the reality: whatever religion(s) (or non-religious group(s)) has the largest pool of young people, is the religious group that in the long run will 'win.'
Three forces always affect church growth: demographics (births minus deaths), conversions (minus defections), and immigration (minus emigration).
Of these three, in most cases, demography (births minus deaths) is the biggest factor. It makes sense when you think about it: the bigger the church, the more babies born will figure into the church, especially as it becomes a greater percentage of the population.
(Think of a theoretical country that is 100% Christian: the only way it can grow is births or immigrants.)
If a particular religious grouping is not having births or immigrants, and another one is, the result is inevitable.
Therefore, one of the big tasks of the church is to help believers marry believers and raise believing children.
(It doesn't follow that large families are essential: having a birth rate roughly equal to the population as a whole, and coupling that with welcoming immigrants and seeking new converts from non-Christians, would do the trick.)