The religion with the most adherents is Christianity. There are 2.1 billion Christians in the world, which is approximately one third of the world's population.
The next most popular religion is Islam, with 1.5 billion adherents, or approximately one fifth of the world's population.
I was interested to see that third place belongs to people with no religion - 1.1 billion people are nonreligious, agnostic or atheist. That's nearly one seventh of the people of the world's population!