Salvian of Marseilles wrote a long hell-fire sermon called On the Governance of God in the 440s which ascribed Roman failures against the (obviously inferior) “barbarians” to their own sins: notably, unjust and excessive tacation, public entertainment, and sexual license.
Some things never change.
Chris Wickham, in The Inheritance of Rome: Illuminating the Dark Ages, 400-1000. (via Matt Yglesias)