I sort of suspect that SOME of Christianity was a glue that managed to hold things together, at least out on the edges. The whole fall of the Roman Empire thing was a horrible mess of strands, not one single thing. When illiteracy is the order of the day, when armed men ride over you (literally), when rapine and slaughter rule, there's little time for art, mathematics, scientific thought. The fact that some small spark was kept alive in some monasteries is, in many ways, remarkable and to be cherished.

Note that in the above I have used qualifiers. The Roman church wasn't the best show in town!

But let's not forget the Eastern Christian churches, or the Coptic Christians, or the Celtic Christian church (until the Synod of Whitby, when the Pope got his way).

I have a sneaking suspicion that Ignorance, Fear, and the like aren't gone, but hiding just out of sight, waiting for their turn again. Read Edward Miller's "A Canticle for Leibowitz."