imo, the dynamics has been, in the western countries many Christians have been losing confidence/faith in terms of numbers and growth, especially when comparing to Muslims.
It might be a natural trend that is irreversible, due to some vital social factors including lacking supply of food/water security
https://en.wikipedia.org/wiki/Hunger , secular education
https://en.wikipedia.org/wiki/Secular_education and the Internet.
When worldwide Muslims can one day be provided a similar level of food/water security, secular education and Internet access, the numbers and growth of (conservative) Muslims would be naturally stablised or even diminished.
Just simple human nature! My 10 cents!
Jesus' theory for "Love your enemies" could be rightfully correct, not only politically!
The total amount for all the above aids in order to provide worldwide Muslims quality food/water security, quality secular education, and quality Internet access could be relatively insignificant, when comparing to any heavy investment on advanced military weapons and armies, through cutting humanity aids!
https://en.wikipedia.org/wiki/Virtuous_circle_and_vicious_circle
LOL