Yeah you might want to look into when Christianity became a unified religio-political doctrine and when the collapse of the Roman Empire occurred. I'm not saying empirically that one had anything directly to do with the other, but I would think that the consolidation of power by Constantine (not the true Christian as he is painted to be) and his successors, despite (and perhaps due to) the schismatic relationship of early Christian groups, led to certain bishops' and then the Church's (as a unified entity) grab for power after the secular empire collapsed and any previous methods of thinking (Aristotelianism, Stoicism, Arianism, Universalism, Platonism, any form of Paganism, etc) were crushed and rejected as heresy, upon pain of death in order to retain power.
Why is any of that relevant to this point? There was no OTHER civilizing influence in Europe at the time was the point. Europe was this mass of warring illiterate superstitious tribes. The idea that something had to CHANGE to bring about the Dark Ages is what is laughable. Europe just being Europe was plenty dark enough on its own.
Yes, the ultimate civilizing of Europe involved creating a CHRISTIAN civilization, as opposed to the former pagan ones, and it became a very great civilization indeed. Heresies were OF COURSE rejected. Sheesh. They were HERESIES. Good grief. And in the end the best of the pagan philosophers WAS preserved, BY THE CHURCH.
So how exactly was Christianity during the period of 400-1400 particularly civilizing, especially in regards to the masses?
They brought them the gospel when opportunity presented, and the gospel brought concern for their fellow man. It took time in such a dark and superstitious heathen continent as Europe but it had its effect and a good one I would say. Too bad it's now being lost in favor of the same old heathen paganisms.