Assuming the death rate of Christians slows down from this weekend's horrific attacks, Pew Reasearch notes that Muslims will outnumber the world's Christians by 2070...
As of 2010, Christianity was by far the world’s largest religion, with an estimated 2.2 billion adherents, nearly a third (31%) of all 6.9 billion people on Earth. Islam was second, with 1.6 billion adherents, or 23% of the global population.
But, if current demographic trends continue... Islam will nearly catch up by the middle of the 21st century.
Between 2010 and 2050, the world’s total population is expected to rise to 9.3 billion, a 35% increase.
Over that same period, Muslims – a comparatively youthful population with high fertility rates – are projected to increase by 73%. The number of Christians also is projected to rise, but more slowly, at about the same rate (35%) as the global population overall.
As a result, according to the Pew Research projections, by 2050 there will be near parity between Muslims (2.8 billion, or 30% of the population) and Christians (2.9 billion, or 31%), possibly for the first time in history.