So I did some more searching. Some claim that Scientology, Mormon, Wicca, et al are the fastest growing religions. As a marketing guy, I can segment data sufficiently to prove most any point. The fastest growing religion might be "none".
There's lots of Islam
PR sites that subscribe to the Islam fastest growing stance. And lots of sites that refute this claim. But so far, I've not come across an independent scholarly study to cite.
Growth
Rate is the statement. I believe the average births per Islam following household is something like 4+, where for Christian households, it's like 1.x. Throw in conversion and it's not out of the question that the rate of growth (not absolute growth) is faster.
Anyway, I'm not a fan of extremes of anything. For me, Timothy McVeigh is no less a terrorist than Mohamed Atta.
Back on point - Years and years ago I tried to rationalize Genesis with Science. Was a day a 100 million years and all that. In the end, Science tries to explain; Faith requires no explanation, just belief.
I honestly don't know. But I do know that most religions try to teach their follower's to be good people - do unto others, et al. In their quest to convince people to be good, a really bad alternative is often explored.
Candidly, I think blind belief that a being created everthing with one wave of his noodle is easy. Trying to challenge that belief while adhering to the "be good people" principles takes courage and intellect.