That is exactly what is going on. The Catholic church is not being able to stop the attrition in regions such as Western Europe and North America (and up to a point, in Latin America). That is why they are investing hundreds of billions of USD in converting the Hindus and Buddhists in Asia to Christianity. The effects are very much visible, in countries such as China and India. South Korea is a Christian-dominated country now.
Then we should apply something like the IIIrd Reich did. Each time you marry you receive something, when you make children, the loan which has been given to you for your first child can be fully kept after the 5th, 6th children, Christians don't pay taxes, or less, being un-married or at least without any child after 35 years old cost you a monthly tax, when you divorce, you have a tax to pay. Then, be sure, people will become Christian again !