As of 2014, the fastest-growing religion in the world is Islam. According to Adherents.com, this is due to the high birth-rates in Muslim countries. Furthermore, unlike other religions, it is a crime called apostasy in Islam to convert to another religion, which helps the growth numbers.
Although Islam is the fastest-growing religion, it is a distant second to Christianity in terms of total numbers of adherents. In 2011, the Huffington Post reported that 33 percent of the faithful in the world were Christian, 21 percent were Muslim, 14 percent were considered atheist or not religious, 13 percent were Hindu, 6 percent were Buddhist and 13 percent were adherents of other religions.