Gibbon was absolutely correct that Christianity was one of the largest causes for the dissolution of the WRE. Unfortunately, he was born before we really knew the extent of the Antonine Plague and Plague of Galen which could have wiped out as much as a third of the population. There was an extreme drop in literacy following the Antonine plague and that's because knowledge was transferred via tutors and apprenticeships, and not a formal educational system. When those tutors, blacksmiths, and workers die there's really no one available to teach the up and coming generation. This is why lorica segmentata disappeared around the 2nd century. The knowledge and infrastructure to mass produce plated armor was lost.
The plagues caused mass conversion to Christianity, which lead to a lot of elites donating their fortunes to the church instead of their children, or to the state. This led to a massively wealthy church, and a very weakened state and populace. After several generations of the church absorbing the wealth, the state could no longer maintain infrastructure and pay for the army. This combined with the barbarians gaining access to Roman technology, equipment, and tactics led to much more evenly matched militaries, whereas prior to this the Romans would face off against largely unarmored barbarian hordes, with only the chieftains wearing armor. Now the barbarians were fielding troops equipped with mail, ballistae, and Roman style weapons and cavalry.
So basically plagues led to Christianity which led to a weakened state which led to the dissolution of the West.
