Jenkins says that at first blush, the answer is obviously Christianity. But when you drill down into the stats, the answer might be Islam:
If Christians point to Africa as the ultimate success story, then Muslims can boast their growing numbers across their historic heartlands. In 1900, for instance, Egypt had perhaps nine million Muslims, compared to 75 million today. In the same period, the number of Iranian Muslims grew from ten million to perhaps 65 million. In 1900, the lands that would become Indonesia had perhaps 34 million Muslims, compared to 190 million today.
Put another way, four times as many Christians are alive today as there were in 1900; but over the same period, Muslims have grown at least seven-fold.