Making predictions based solely on population changes is a dangerous game, since other factors can intervene, but it's one of the few games we have. Based on birth rates in various communities and shifting world demographics …
… By 2050, Muslims are expected to be close to the same proportion of the world population — around 30% — as Christians will be. Both religions will be growing in numbers, largely in sub-Saharan Africa.
… In the US, Muslims will supplant Jews as the largest non-Christian sect, growing to about 10%, with Christianity dropping from about 3/4 to 2/3 of the population, and "Unaffiliateds" growing as a percentage.
… "Unaffiliateds" as a world population, will decrease in proportion, due to higher birth rates among most religious groups.
Whether this is good news, bad news, or just news to bear in mind is left as an exercise for the reader.
The Future of World Religions: Population Growth Projections, 2010-2050
As of 2010, nearly a third of the world’s population identified as Christian. But if demographic trends persist, Islam will close the gap by the middle of the 21st century.
Yeah, I remember reading this a few months back and the problem I had with it is that it's based solely on birth rate and not on what the current pattern of people just becoming "un-affiliateds".
Of course, this assumes that all children will adopt their parents' religion. Anecdotally most of the unaffiliateds I know (myself included) had religious parents, so I don't think using population projections really applies to what's going to happen to that population.
Well, I caveated the risks of just going with population growth trends in the first sentence. But the growth of "Unaffiliated" seems to be most substantial at the moment in the US (and, ahead of the US curve, in Europe), and not in the highest birth rate areas. I'm not aware of any Unaffiliated trend in sub-Saharan Africa or in India, for example. There could be a weather change / tipping point there, but I don't see it in the next 40 years.
The article actually goes into this.
I don't think it's unfair to say that, in most of history, and in much of the world today, religious affiliation still tracks from parents to children as a general rule, for better or worse.