The same kind of Christianist attributed the decline of the Roman Empire to lingering paganism and way too much bad sex.
Of course, pagans attributed it to the growing power of Christianity and neglect of duties to the (true) gods, but who listens to them?
A much better story is massive and colossally destructive barbarian invasions.
But it's tough to spin that.
And today the West is not declining, but lots of other folks are rising as their economic power grows, relative to ours.
Thank decades of free trade and deliberate global leveling.
And maybe most of all thank the conversion of the Chinese Communist Party to the capitalist road.