Sociologist Peter Berger has a theory about why religious believers tend to have more kids than their secular peers:
I will venture a hypothesis. Religion has always given its adherents a sense of living in a meaningful universe. This protects individuals from what sociologists call anomiea condition of disorder and meaninglessness. Religion, by the same token, gives a strong sense of identity and confidence in the future. More than anything else that human beings may do, the willingness of becoming a parent requires a good measure of confidence in the future. Mind you, this is not an argument for the truth of religion. Illusions may also bestow meaning and confidence. But my hypothesis offers an explanation for the ubiquity and persistence of religion.
I am not sure whether this function of religion works in the same way in the Abrahamic traditions (Judaism, Christianity and Islam) as in the religions to the east of the Muslim world, notably Hinduism and Buddhism. It probably does. 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 mothers comforting words.