See this report published on December 18, 2012:
The Global Religious Landscape
A Report on the Size and Distribution of the World’s Major Religious Groups as of 2010
Pew Forum on Religion and Public Life
Every religion is a minority religion. Whatever a person’s religious beliefs, most religious people in the world have beliefs that contradict that person’s beliefs.
Christianity in its entirety is a minority religion, with about 32% of the world’s whole population, and about 38% of the world’s religious population. (And, of course, there are many incompatible beliefs within Christian denominations, with Roman Catholics about 50% of Christians).
Islam is the second largest religious grouping, with about 23% of the world’s whole population, and about 28% of the world’s religious population. (And it is split especially between Shias and Sunnis, the “sides” of the longest running feud in human history).
The “unaffiliated”, which includes atheists and various other groups, are about 16% of the world’s whole population, ahead of Hindus at 15%.
Other religion-related groups are much smaller. Across the world, 1000s of gods are worshiped and 1000s of religions are practiced.
In Europe and North America Christians are on average older than the unaffiliated. (In Europe the median age of Christians is 42 and of the unaffiliated is 37. In North America the median age of Christians is 39 and of the unaffiliated is 31.)
So in these regions the people dying of old age are more likely than average to be Christians, while the people being born are more likely than average to become unaffiliated. There is an inexorable increase in the proportion of the unaffiliated and decrease in the proportion of Christians generation by generation. There is nothing to stop this trend.