As Spengler pointed out with others -most notably in agreement with Gibbon - Rome has lost 90% of its population between the Augustininan age and by the time the Huns appeared at the gates, turned back miraculously by the exhortations of the Pope.
Gibbon attributes the decline to Christianity, but it is not the generally accepted view, most moderns favouring economics to transcendence. Nevertheless, Gibbon is mostly right, although not in the way he himself understood his argument.
Christianity sapped the strength of the pagans, not the christians. Without the Faith, they lost the will to live and reproduce, having no children, they disappeared and the Christians won by default, they were the last men standing. As the Lord himself taught, few are the saved and many are the lost to perdition. So, the ten percent of the population kept the commandments, was fruitful and multiplied, and by the time of Attila, they were the only ones left.
Something similar is happening today. The west's post-christian - or neo-pagan - population is rapidly disappearing, and only the real faithful christians and the muslims who never accepted modernity will be left in the future. The secularists and faithless are doomed to disappear.
Thy will be done!