Christianity became powerful a couple centuries LATER. This "couple centuries" was the period as long as from American Revolution to this day. Christainity was BORN at the time of Roman decadence and spread of witchcraft, but it was among distant and distinct Jews. Only later Christianity took over and transformed decayed pagan society. If there is the causal relationship it is one more step detached: first was the collapsing order then second was spread of witchcraft and despair and the third was Christian repentance.
Certainly I'm not blaming Christianity for that, but it seems just as silly to blame "paganism" on the fall of Rome, as it was around for hundreds of years before hand.
It the text the words used were - "symptom of a collapsing order". This collapsing order was pagan and contained some pagan natural virtues. The collapse was connected with the corruption of the good aspects of old Roman religion into the witchcraft, decadence and perversion.
To repeat - Christianity prevailed LONG AFTER the Republic died.