They are therefore already taking advantage of it, but I wonder when it will be removed it the growth differential will equalize.

My guess is that it won't, and the religious represent a very significant percentage of the population - until they become the majority.

Without willing to insult anyone, I wonder if a human group growth with religion, they reach decadence with atheism - then stats again, eventually with a different religion, in a loop.

EDIT: the point is not about "cultural group" but religion. Are these minorities becoming atheist, or do they keep some religious self identification? I guess they still identify as such, even if they don't practice. Culture is hard to bring back, but religion can grow back very quickly