Hmm PeterClough – not that I'm one to confuse matters BUT
1 = given that the Ostrogoth barbarians invading Rome were Arian Christians coming (allegedly) to save Rome from a different bunch of barbarians (haven't we heard that recently) – you could argue it was a bunch of Christians saved by another bunch of Christians.
2 & 3 = the ill educated Roman Christians who sacked/raped/pillaged Constantinople as part of a Crusade (3rd or 4th?) hastened the end of said civilization leading to 3. Importantly this then led the knowledge stored in Constantinople to flood into western Europe educating the Roman Christians (a minor event called the Renaissance)
4. Agreed.
Finally Dark Ages, what Dark Ages, I believe the term is early Medieval Period…