I investigated this heavily the first time that I heard it many years ago.

From non-sectarian sources I learned that Islam appears to be the fastest growing religion for a number of reasons. For example, it is, in fact, the fastest growing religion in Europe. Why? Due to IMMIGRATION.

More importantly, the birth rate of Muslims is nearly twice that of non-Muslims. Most Muslims live in Asia (particularly Indonesia) and Africa. In those countries, people are more likely to have huge numbers of children than in the West. Moreover, the majority of these new Muslims are infants. In the majority of cases, they are not allowed to choose a religion so if a Muslim person has a child, that baby is considered a Muslim and will be counted in the number of adherents.

As far as religious conversion goes, Christianity is the fastest growing. Evangelical Christianity is making great inroads in places where one cannot obtain an accurate number like the Middle East and China. Most converts to Islam are women marrying Muslim men, not converts due to a faith-based reason. Even with all of the black Americans seemingly flocking to Islam, their numbers are fairly insignificant as a percentage.