Steve Bruce of University of Aberdeen wrote an article in the Scotsman about the decline of the Church of Scotland, aka the Presbyterian Kirk. He goes through declining numbers of adherents, then the reaons and options. His closing paragraph:
People do not accidentally become religious. Being a Christian is not “natural”; it is an acquired characteristic. Like a language, it must be learned and, if it is not used in the home, in everyday conversation and in public life, it dies out. As the population that speaks a minority tongue shrinks, decline does not slow; it becomes faster. There is no natural obstacle to the death of a language. I do not see why the fate of a religion should be different.
I think his analogy is false. The motivation to learn a (dying) language for those who were not brought up in it is different than joining a spiritual and religious path. I do not have a good handle on numbers and trends, but I see religion as a growing force in the world, while lingusitic differences are diminishing.