Population is not really a problem. The challenge is pollution per capita.
Ex: A religious person goes to a developing country, promotes birth control, and comes back to be rewarded for preventing excess births to the degree of 100 children. They then proceed to have 2 children of their own. Given that the developing country has a footprint per capita two orders of magnitude less than that of the proselytizer (i.e. 5 versus 800, to use circa 2000 numbers), that person has in effect only slightly mitigated the cascading impact of their own actions.
It's more about raising everyone alive not just to our collective higher standards of living but also in reducing footprint.