Not following your thesis about Christians not reproducing. Only non-Christians are having large families?
Of course some Christian families are reproducing. But at a population level, say the US as a whole or western Europe as awhole the numbers are headed down. Overall there are more Christians than muslims in the world right now. That changes at about 2050 if not sooner. Pew has a reasonably good picture of the trends if you wish to google it.