Ask Lemmygrad

This belief came about simply because the religious sects that didn't promote reproduction died out.

For instance, Paul the Apostle advocated that celibacy was superior to marriage. Since early Christianity was an apocalyptic branch of Judaism that believed the end of the world would happen within believers' lifetimes (at which point Jesus would return to judge all), Paul considered it most important for believers to maintain absolute purity in preparation for judgement.

When that judgement day didn't come in time, sects that were celibate would obviously grow slower and be outcompeted by sects that advocated childbirth. This has created the religious landscape today, where new children are the predominant way religions grow.

Some sources:

• https://youtu.be/SwohpJU1Tco