I asked the guy who knows about this stuff, the very competent Eric Kaufmann, whose book Shall the Religious Inherit The Earth has captured my attention for quite a while, this question on Facebook:
"I am interested in how the demography of Eastern Europe will affect the global Eastern Orthodox population (230 million). 93.2% of Eastern Orthodox live in 20 countries of Eastern Europe and Greece. The average TFR of these 20 countries is 1.37. The top 82% of Orthodoxy is located in Russia, Ukraine, Romania, Greece, Belarus, and Bulgaria, which also have a combined TFR of 1.37. With fully half of the world’s E.O. in Russia (TFR 1.42).
What I am interested in is if there are higher fertility groups of Orthodox within these nations that will outbreed their low fertility neighbors? The data is hard to find though. Orthodoxy seems set for an unusually steep decline in population compared to Catholics, Protestants, or most Muslim groups. Am I right about this?"
"Yes, I think that's right. Though I believe Orthodox attenders have, as elsewhere, somewhat higher fertility than seculars or nominals (see Tom Frejka's work[*] on European survey data on this). What they are lacking is a closed, fundamentalist group with high fertility a la Dutch Calvinists or Finnish Laestadian Lutherans or Orthodox Jews."
Eastern Orthodoxy has it's population center of gravity in Eastern Europe (including Russia) and Greece. So the demography of Eastern Europe will affect the global Eastern Orthodox population (230 million) in the future quite heavily. This is of course assuming there are no large sub populations of Orthodox with high fertility rates. I had not heard of any, and Kaufmann seems to confirm that there are not.
Of the factors that affect religious population positively the 2 biggest are:
1. Growth from within. Religious populations within countries with high fertility rates due to prolonged demographic transition, eg. Africa and Latin America. In these societies, large, young populations of adherants (even nominal ones) will still have a much higher fertility rate than their counterparts in countries further into their demographic transition.
2. Endogenous growth sects. These are the groups Eric calls "closed, fundamentalist groups". So this is not only the Amish, Hutterites, and Ultra Orthodox Jews, but less closed (yet still self consciously seperate) groups like the Mormons, Quiverfull type Protestants, and conservative Catholics. Each of these groups is closed to some degree from worldly influence and has intentionally high fertility rates while rejecting contraception.
On the flip side, nations who are far into the demographic transition have very low fertility and high secularization, with aging populations and actual population decline... right now. Not future decline, but right now decline due to people not procreating.
The other major Christian groups have eggs in all of the above baskets, and have more geographic diversity as well. Orthodox have none. Somehow Eastern Orthodoxy has managed to be most populous in the lowest fertility countries on earth, So while Catholics in Italy (1.4) and Spain (1.48) do have a fertility rate just as bad as in Ukraine (1.29) or Greece (1.39), there is the crucial difference that Catholics are not only in Italy and Spain, while demographically, it can be said that Orthodox are only in Eastern Europe. Sceptical? Look at my research in this chart: