The Roman economy required continual expansion of the empire. They managed this by superior military: disciplined, well equipped, full time, professional armies, which most European tribes of the time lacked. To pacify the conquered peoples, the Romans had to let them adopt the urban lifestyle of Rome and share in the wealth of the empire, and for this reason the Romans had to find new territories to exploit for grain, metals, slaves etc. In effect the conquered, exploited people later became the conquerors and exploiters of others. When the empire could not expand further, when it ran up against other empires in the middle east and unconquerable tribes/terrain in Germany and the Russian Steppes, it could no longer afford the expense of maintaining the armies necessary to defend its borders and it collapsed. Trajan expanded the empire to its largest extent, and was followed by Hadrian, who wisely saw that it was all downhill from then on and fortified the borders to delay this process as long as possible.

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Discussion at the BBC’s Thinking allowed podcast

While you are almost certainly right and its just a hunch, there does seem to be something perniciously monomaniacal about monotheism and its spread from Middle Eastern origins, coincident with Rome’s decline.

From the account of religion in the Decline and Fall, it sounds like Roman paganism lead to a kind of pragmatism, where everyone’s gods were adopted and many opinions tolerated - this wouldn’t have necessarily created a religious environment which was antithetical to the spread of rational ideas amongst the plethora of superstitious ones.

The single mindedness of a single god and religious manual and the rigid crystallization of its interpretation at the Nicaean Councils might have created an environment less tolerant of other competing ideas such as evidence based reason.

i would not agree with that personally, since the romans (or those defined as romans) were continuously superstitious throughout their entire history.

Rather like the feedback loop from settlements, which enabled infrastructure developments, which enabled further growth in settlements, I wonder if the decline created religiosity and superstition which further exacerbated decline through a collapse of reason.

the main cause was vastly increased agricultural production due to the aggregation of farming land into the hands of the few, creating the huge latifundia estates. this destroyed the back bone of the republic - the free citizen farmer. the proletariat roman citizenry flocked to (you guessed it) the cities. so rome’s initial explosive population growth can be seen as a combination of growth via increased food production and the citizenry leaving the countryside for the urban areas.

the sharp population drop off may coincide with some pivotal moments in christian history, but as we all know, correlation does not imply causation. the orthodox eastern roman empire continued as a late roman entity into the 7th century, and as a political entity till 1453.

the gradual decline from peak may be attributed to rome’s decreasing importance as an imperial center. rome eventually lost its capital status to ravenna.

the sharp drop off right after the 400AD mark was caused by a pivotal moment in roman history - the sack of rome itself by alaric and his band of merry visigoths. a few decades later, rome lost north africa to the vandals. north africa was the breadbasket of the west, since latifundia had long stopped producing grain for eating, preferring to create goods for export. a little while after that, rome was again sacked by the vandals. more data points might have captured the population shifts caused by these events. but essentially, war + no food = no romans in roma.