The Christians were part of the fall, not the cause of the fall. Dictatorships work well when there's a good dictator. And not all of the Roman Emperors were terrible. They could have kept on keeping on, had they not torn themselves apart. The downhill slide began far earlier than the Holy Roman Empire. And the rise of the Christians were, in part, a step along the slide that helped to ram things into the ground, but certainly they didn't cause the fall. They were simply offered the fruit, with several bites already taken out of it, and helped Rome, like the serpent, bite the dust.
The last line "... helped Rome, like the serpent, bite the dust." is clever as hell. Though it requires knowing that in Genesis the serpent is punished by being forced to move on its belly and eat dust. The tangled sort of metaphor may simply be brilliant using Genesis' Fall of Man as a metaphor to the Fall of the Roman Empire. And the comparisons of Adam being offered fruit that was already eaten a bit, with the Christians taking over an empire already in shambles and just making it a bit worse. The whole post might be worth reading, but that end part is just clever.