There may be a number of reasons why theist people have more children than non-believers, as Anthony Gottlieb–former executive editor of The Economist and author of The Dream of Reason–suggests in this article from Intelligent Life.
Like other demographers, Eric Kaufmann expects western Europe to become markedly more religious in the course of the 21st century, as a result of the relatively low fertility of unbelievers and immigration from more pious places. Not only do denominations with traditionalist values tend to have higher birth rates than their more liberal co-religionists, but countries that are relatively secularised usually reproduce more slowly than countries that are more religious. According to the World Bank, the nations with the largest proportions of unbelievers had an average annual population growth rate of just 0.7% in the period 1975-97, while the populations of the most religious countries grew three times as fast.