If Christians would do what God said, “Multiply and replenish the earth,”, the Lord Jesus could fix our civilization very quickly.
Muslim birth rates are badly skewed in that the death rate of those same infants is excessively high, many do not make it to childhood or adulthood.
China is also reaching a demographic nightmare without noticing that at least 10% of their peasantry has become Christian and is flying under the radar with their children.
http://berkleycenter.georgetown.edu/events/why-christianity-is-thriving-in-china-today