This may be unpopular, but I already feel the earth is well beyond it’s ideal human population capacity. We are straining resources because of our bad policies and practices and politics.
It's obviously very difficult to predict, but there's also some expert speculation that affluence will spread widely enough to slow the birth rate down to be close to the death rate and we'll reach some sort of equilibrium.
Those religious minorities are quite small right now, but if they manage to sustain their current growth into the coming decades, they may end up being a much more substantial component of the population.