Bottom line: both Christianity and Islam are growing faster than the world population (so the world is becoming more religious); and Islam is growing faster than Christianity, so the % Muslim in the world is increasing.These numbers are based on projections made by the Center for the Study of Global Christianity at the Gordon-Conwell Theological Seminary in Massachusetts. Although they use UN Population Division projections for their baseline numbers, they clearly have to make their own estimates of the population by religion. Their data project that by 2050 there will be 3.4 billion Christians, accounting for 35% of the world's projected population of 9.6 billion. By comparison, they project a total of 2.7 Muslims in 2050, representing 28% of the world's total.
Meanwhile, last year Pew Research published its projections of the world population by religious affiliation. They projected a total of 9.3 billion people in the world by 2050, of whom 2.9 billion are projected to be Christian (31%), and 2.8 billion Muslims (30%). So, we see that the Christian group making projections has a higher number for members of its religious group than does the secular research organization. Does that surprise us? I think not. In either case, the conclusion stands that largely through birthrate differences, the Muslim population of the world is growing faster than the Christian population.