I just linked a couple of articles from the Pew Research Center, but I wanted to comment on this one: The Future of World Religions: Population Growth Projections, 2010-2050.
Here are some highlights, assuming trends continue to 2050:
The number of Muslims will nearly equal the number of Christians around the world.
Atheists, agnostics and other people who do not affiliate with any religion – though increasing in countries such as the United States and France – will make up a declining share of the world’s total population.
In Europe, Muslims will make up 10% of the overall population.
In the United States, Christians will decline from more than three-quarters of the population in 2010 to two-thirds in 2050, and Judaism will no longer be the largest non-Christian religion. Muslims will be more numerous in the U.S. than people who identify as Jewish on the basis of religion.
Four out of every 10 Christians in the world will live in sub-Saharan Africa.