Sea Breeze,
That has got more to do with fertility rates than anything else
The Future of World Religions: Population Growth Projections, 2010-2050 | Pew Research Center
Islam is by far the fastest growing religion in the world, simply because Muslims out-breed everybody else by a long shot! (Assisted, too, by the fact than in at least 13 of the world's countries, it is a capital crime to be an atheist).