Plagues and floods took their toll. More crucially, the aqueducts which had formed the backbone of the efficient and complex water supply system that served most of Rome fell into disrepair after barbarian raids, leading to the population shrinking towards the river Tiber and around the Campus Martius, and falling to less than 50,000 people. Into this situation the popes of Rome stepped in, creating a serviceable network of churches and ecclesiastical institutions in order to govern the city. The city was thus introduced to Christianity and soon laid claims to being the most important Christian spiritual center.
But the Church was not alone in wanting to govern Rome. By 552CE, Rome came to be occupied by the Byzantine empire, and thus began a conflict of interest between the church and the Byzantine regime that was to last for the next few centuries. Under the Byzantine rule, trade, commerce and administration deteriorated, and Rome became a shadow of its former self. The Byzantine emperor Constans II visited Rome in 663 CE, only to strip the city of all its metal from buildings and statues in order to make armaments.
The Byzantine city of Constantinople soon emerged to usurp all the commercial successes of Rome, and its former influence in trade in Europe and Asia. With the decimation of the aristocratic class of Rome, a huge market of luxurious goods eventually disappeared taking away prosperity from the common people of Rome, many of whom had found jobs in the trade or manufacture of luxury items.
A lot of the famous Roman skills in artisanship also disappeared due this lack of market. More and more people began leaving Rome. Trade in Rome also suffered due to insecure road and maritime travel at the mercy of Germanic and barbarian tribes in what was left of the mighty Roman empire.
After Pope Gregory I took over in the 6th century and began to exert more influence than the Byzantine imperial exarchs, the populace of Rome got divided into the clergy, nobility, soldiers, and the lowest class, thus introducing a sphere of Christian influence in socio-political scenario in Rome.
But the Church and its members that wanted to govern Rome along with the Byzantian empire were more interested in self-aggrandizement than in the benefit of the city itself, having become too used to the luxurious ways of the erstwhile Roman elite. Even before the fall of Rome, the popes had concentrated their energies in developing their own domain at the eastern edge of the city far away from other Roman habitations, called the Lateran.
It is to Lateran that the church limited most of its activities in Rome largely ignoring the disrepair to which the city gradually fell. The pilgrimage to the shrine of St. Peter, where the saint's holy remains were supposed to have been interred, became a sort of commercial activity around which a suburb developed after a fashion, but even the visiting pilgrims who came to pay their homage failed to make any difference to the state of Rome and its gradual decline.
Even after the suzerainty of Rome to the Byzantine empire was formally over in the 800CE with the crowning of Charlemagne as Holy Roman Emperor by Pope Leo III, the papacy was not able to consolidate its hold over the city, and the Arabs sacked