Not sure if Gibbons thesis about Christianity bringing down the Roman Empire is correct...
The empire was on the decline well before Christianity was becoming a dominant force. From 235-284, the Roman Empire was engaging in civil war. Contenders for the throne bribed soldiers to become the next emperor. Although, out of 22 emperors, there were only two that didn’t meet with a violent or deadly demise.
At the same time, Sassanid Persians were invading Rome and Valerian was captured and held in captivity until he died. German tribes were flooding into the empire, disrupting boundaries by moving into Greece and Asia minor.
Aurelian tried to restore the empire by building new walls, however war and invasions had already taken their toll on the government. So where were the Christians at this time and what impact were they having? They weren’t present, nor did they have a hand in the invasions and civil war.
Economically, farm production was deteriorating as fields were ravaged by barbarians and by Romes own armies.
The Romans confiscated produce and livestock from farmers, therefore they themselves directly contributed to the failing economy.
Gold coins were disappearing from circulation, while silver was being diluted causing inflation. To put it simply, the economy was being depleted by invasions and social issues, excluding religions that were monotheistic and were hardly having an impact of the fall at the time.
Again, what did Christians have to do with these?
I think Christianity glued the Roman Empire together and let it last a few more years. This was part of Constantine’s motivation in establishing it. It also gave some cohesion to the Germans and, after Rome finally fell, to Europeans in general. Religion can be a powerful instrument in controlling the masses.