I thought the chart above, from the Pew Research Center, was interesting -- and thought it was fitting to post it on this religious weekend. Currently, christians make up about 31% of the world's population, while muslims make up about 24%. But by 2060, with muslims growing at a significantly faster rate. they will make up roughly equal percentages of the world population -- with christians being about 32% and muslims being about 31%.
I think that is probably true, since the estimates are figured on birth and death rates -- and those religions (like all religions) depend mainly on people being born into the religion for their growth.
The one area that I disagree with is the projected growth of the non-religious. They project the percentage of non-religious will fall from 16% to 13%. If you look at only the birth and death rates, that would be true. But the growth of the non-religious has never depended on both and death rates.
The non-religious are those who accept science -- including contraception to plan their families. While the growth of the non-religious has been larger with each generation in the United States (and the world), that is not because they are having more babies than the religious -- in fact, they are probably having less. Their growth is because when children mature into adults they think more deeply about religious beliefs -- and a growing percentage discard those beliefs.
This has been true of a growing percentage in each of the last several generations, and there's no reason to believe the percentage of the non-religious won't continue to grow in future generations.