The Church was forced to fall back on its own internal resources - most of the people of the West were just emerging from barbarism and had inadequate legal systems and almost zero literacy to start with.
The only sustaining cultural force was Christian theology - if Church itself did not dictate it's own discipline and mores, what did? The Vikings?
It could be just as easily argued that the survival of the Eastern Empire allowed the faith there to flourish in a way that was not possible in the chaos of the fall of the West.
Except, of course, that the faith gradually but relentlessly contracted in the East, it didn't flourish. Christians probably reached their high watermark as a percentage of the Eastern Empire's population during the reign of Justinian.
"Except, of course, that the faith gradually but relentlessly contracted in the East, it didn't flourish. Christians probably reached their high watermark as a percentage of the Eastern Empire's population during the reign of Justinian."
Well, there are many reasons for this, but I think the "numbers" game is a dead end. The Church may have become smaller as a percentage of population, I honestly don't know, but it certainly became more fervent and I doubt anyone would argue that its theology didn't flower with men like +Symeon the New Theologian and +Gregory Palamas. In any event, didn't the Pope recently talk about the Western Church becoming smaller, contracting, but becoming thereby more vibrant and faithful to Holy Tradition?