After "winter" always comes "spring", after decline of the Western Faustian rational Civilization (winter), there would be the Springtime for new Culture. Rationalism will fade out and the realm of pure primitive religion will become visible.
In Spengler's cycling concept of history Spring stage is characterized by high level of religiousness and spirituality. The fact he believed, that Russia was about to come to its Spring was, probably, the ground for him to conclude that it will be spiritual savoir of the world.
Spengler, Oswald, and Charles Francis Atkinson. 1926. The decline of the West. New York: A. A. Knopf.