Pew Research has a new report out projecting the global growth of population by religious affiliation:
While the world’s population is projected to grow 32% in the coming decades, the number of Muslims is expected to increase by 70% – from 1.8 billion in 2015 to nearly 3 billion in 2060. In 2015, Muslims made up 24.1% of the global population. Forty-five years later, they are expected to make up more than three-in-ten of the world’s people (31.1%).Westerners tend to think of Muslims in terms of the Middle East, but the Pew projections suggest that Indonesia will continue to be the most populous Muslim nation in the world, and that the second largest population of Muslims will be in India, despite their continuing to be a minority in that Hindu-majority nation.
You should be able to figure out the reason for the fact that Muslims will outnumber Christians by mid-century. It's fertility. Muslim women have more children on average than non-Muslim women (see graph below), and the younger age structure of Muslim nations means that there are many more women of childbearing age having those numerous children.