It is a statement that really means nothing but somehow it is marketed as “evidence Islam is right!” Let me remind them that at one time, ancient Egyptian religion was the fastest growing one, this was beaten by the Roman and their fastest growing religion, and if you look East, Buddhist and Hindu was the fastest growing religion. And in Americas, the Mayans and that Incas were the fastest growing religion. And it was after Islam, Christianity was the fastest growing religion in Africa and Americas carrying the cross that in Islam we reject. Under Islamic view, all these religions are wrong and not true….yet at one point or another, they were the fastest growing religion. And not to put you down or anything, the reason why many of these studies that suggest Islam is the fastest growing, is because they do not count Atheism or Agnosticism as religion. If included, you will be very disappointed to hear that in fact, it is these two the fastest growing faith (or lack of faith).
Have a nice day.