According to the Pew Research Center, Christians were the largest religious group in the world in 2015.
In the 2017 report, "The Changing Global Religious Landscape," the Pew Research Center states that there are roughly 2.3 billion Christians in the world. That figure translates to slightly more than 31 percent of the world's population.
Muslims (1.8 billion) make up the world's second-largest religious group in 2015, accounting for nearly one-quarter of the world's overall population of 7.3 billion. Pew researchers project that Muslims will be the fastest-growing major religious group between 2015 and 2060, followed by Christians, Hindus and Jews, respectively. That might be because so many subscribers to the Muslim faith are young. In fact, Muslims have the youngest median age (24) of any of the world's major religions, meaning many Muslims have their child-bearing years ahead of them.
The median age for Christians is 30, which could be why the Christian population is not expected to grow as significantly in the coming decades as the Muslim population.