In an update in April 2017 of a post that was originally published on 23 April  2015 PEW Research Centre has predicted

that during  the next half century by 2060 Islam will surpass Christianity as the Worlds largest religion.

Muslims will grow more than twice as fast as the overall world population from the current 1.8 billion to nearly 3 billion in 2060.

The main reasons for Islam’s growth ultimately involve higher fertility, young median age and religious switching or conversion.

As an example India’s number of Muslims is growing at a faster rate than the country’s majority Hindu population, and is projected to rise from current 15% to 20% (or 333 million people) in 2060.