Eric Kaufmann, in Shall the Religious Inherit the Earth, explores what he calls the "soft underbelly of secularism: demography." Secularism has been growing in all developed nations, and there are secularized pockets among the educated in all countries. Northern Europe, especially, has gone a long way down to the road to irreligion. Since the Enlightenment began, intellectuals, both secular and religious, have been predicting the decline and disappearance of religion.
Kaufmann points out, though, that even in the most secularized society, secular people do not have enough children to replace themselves. In most societies, the moderate or mainline religious groups also have sub-replacement fertility. On the other hand, fundamentalists in every religious tradition have enough children to grow - some of them by gigantic accumulating rates.
Moreover, secularity grows by conversion, mostly from the slightly or moderately religious. The strongly religious, by contrast, typically build strong religious communities to go with their firm faith, which helps them retain their children.
The numbers from American Protestants can represent those from other countries and religious traditions. For a population to be stable and replace itself, each fertile woman needs to have, on average, 2.1 children. This is the magic number of the Total Fertility Rate (TFR). The TFR of secular and Protestant Americans:
Moderate Protestant: 2.0
Conservative Protestant: 2.5
Kaufmann predicts that secularists will continue to grow as a proportion of the U.S. population to mid-century. Then, though, the higher fertility and higher retention of religious conservatives (not all of them Protestants, of course) will catch up and become a larger and larger portion of the American population.