World Religions by Population
A breakdown of global religious adherence and growth trends
Islam comes as a close second to Christianity with about 2B adherents. Hinduism is in 3rd place with 1.2B adherents. Non-followers of any particular religion come in 4th place with 1.1B people, and Buddhism comes in 5th at 500M adherents. Every one of these populations is growing, except non-followers (which is declining) and Buddhism (which is expected to stay the same).
It's worth noting that there are about 8.1B people on earth, so this tally misses about 700M people. Those comprise folk/indigenous religions (400M), other organized religions (100M) like Sikhism (25M), Judaism (14M), Jainism (4M) and Zoroastrianism (200K), and then unclassified religions make up the final 200M people (e.g. Pastafarianism, Realism, and Scientology).