I read... somewhere, it may have been Cahill, that early Christians largely considered Rome to be the specific earthly reservoir for Christianity and that Christianity was only meant for places where their god had decided the Roman Empire should include. I don't recall how Ethiopia or Armenia were supposed to fit in this theory, but Ireland does seem to have been one of the first areas to feel the effects of missionaries, a sort of testing ground for conversion techniques.
So... keep the Western Romans around for a while longer and make Christianity seem not just foreign but the arm of a homogenizing and conquering empire? Then again, how much longer could the West hold out, realistically?
Of course, this would involve taking my source at face value, so yeah.
I'm so sick of hearing of this biased, short-sighted theory that a non-Christian post Roman Europe would be going down the shitter in cultural terms. Its utter bollocks. Sophisticated, literate, urban societies managed to appear without the guidance and influence of Christianity. Why shouldn't the same thing not happen again? The Germanic tribes that took over were no less intelligent or organized than any other ethno-cultural group that appeared before them. As long as they're engaged in long-distance trade with other cultures, minting their own coinage, have full use of an alphabet (Runic, Ogham), then "civilization" should spring right back.
Honestly, I can see exactly one civilising advantage of Christianity and that's the common cultural background available for the spread of intellectual and technological developments. Of course, I'd imagine persecuting heretics and purported witches does its own bit to stunt growth...