Gibbon is to be respected as a historian, but his assertion about Christianity’s “weakening influence” on the Romans has to be taken with skepticism. The Eastern Empire went on for a thousand years after the West “fell”, and was Christian the whole time.
One of the interesting things about Rome, at least as near as I can tell, is that it really didn’t have a history of ethnic exclusivity. It readily absorbed the Latin tribes that really weren’t “Roman”, then the Etruscans, the Cisalpine Gauls, the Illyrians and Pannonians, later the transalpine Gauls and even a lot of Germans. It wasn’t “immigration averse”, and tended to Romanize diverse peoples. Some of the emperors weren’t “Romans”. Its armies included a lot of diverse people almost from the beginning.
The Empire was always a struggle, and it missed destruction by near misses a number of times. But there is no question the population declined relative to that of the peoples beyond the Rhine and Danube. Some blame birth control and abortion for that, resulting from increasing penury among the citizenry. Some blame malaria. Some blame infertility due to the pervasive use of lead piping for water. Some blame the move to the East. Probably all are correct to one degree or another.
