Peter Berger explaining his hypothesis as to why religious people have more children than secular people (here):
For a believing Jew, Christian or Muslim, the future of the world, his own future, and that of his children lies in the hands of a compassionate God. Every mother, of any faith or of no faith at all, will get up in the night to comfort a crying child. She may not speak. Her presence and her holding the child may be enough comfort. If she does speak, it is likely to be some variation of saying “everything is all right” or “everything will be all right”. This may well be true at the moment. In a purely secular perspective, these formulas are finally not true. The mother, the child, and everyone and everything they care about are fated to perish. Religious faith gives a cosmic validation to the mother’s comforting words. It is no accident that the most famous lines of Julian of Norwich, that elusive medieval mystic, are reminiscent of a lullaby: “And all will be well. And all will be well. And every manner of thing will be well”.
Which is reminiscent of something Stanley Hauerwas has already written (in Resident Aliens: Life in the Christian Colony):
We have children as a witness that the future is not left up to us and that life, even in a threatening world, is worth living—and not because ‘Children are the hope of the future,” but because God is the hope of the future.
If we lack good reasons for having children, we also lack good reasons for deciding not to have them. Christians are free not to have children not because of most contemporary rationales (‘I don’t want to be tied down.’ ‘I would not bring children into this messed up world.’), but because we believe in the power of God to create a people through witness and conversion rather than through natural generation. The church must be created new, in each generation, not through procreation but through baptism.
It is our privilege to invite our children, and other’s children, to be part of this great adventure called church. Christians ought to ponder what an amazing act of faith it was for Jews in the face of constant and death-dealing Christians and pagan persecution to go on having babies. People of God do not let the world determine how they respond to tomorrow.