Demographics. It is a fact that populations who are more "religious" tend to have more children. In this story in the Winter 2008 issue of INTELLIGENT LIFE magazine, Anthony Gottlieb suggests that something else might also be going on [HT: Christopher G.].
Does having more children also tend to make people more religious? I confess that the more children we have had, the more important church and theology have become. I think this author might be on to something. He cites anecdotal evidence that secularization tends to follow demographic declines in fertility, not visa versa.
But what, then, causes the decline in fertility? In a previous post here, Oswald Spengler offers some historical insight. Cultural attitudes about children come first. Cultural attitudes tend to infect even the religions themselves. Look at what happened over the past century in Christianity, and what happens to Muslims who live in post-modern cultures. And, even though the Roman Catholic church maintains its official position against contraception, the majority of those who claim to be Roman Catholic tend to contracept right along with the rest of the culture they live in. Gottlieb suggests that the resultant decline in fertility then brings about a population's gradual abandonment of religion.
Now, other religions aside, let us consider what correlation faith and fertility could have for Christians. Does having more children make one more faithful? I think not. Faith comes by hearing, not by procreating. Faith is created, maintained, and strengthened by God's Word and Sacraments. Children certainly are blessings, but they are not a means of grace. Scripture would even suggest that those who have the gift of celibacy can be even more devout. However, consider this: perhaps contraception and faith are not very compatible. Unrepentant sin chases away faith and the Holy Spirit. I should hasten to add that I do not mean to imply that all use of contraception is unrepentant sin.