The Bishop of Rome, called the Pope, was important since the early days of Christianity because of the martyrdom of both the apostles Peter and Paul there. The Bishops of Rome were also seen (and still are seen by Catholics) as the successors of Peter; he being the first Bishop of Rome. The city thus became of increasing importance as the centre of the Catholic Church. After the fall of the Western Roman Empire in 476 AD, Rome was first under the control of Odoacer and then became part of the Ostrogothic Kingdom before returning to East Roman control after the Gothic War, which devastated the city. Its population declined from more than a million in 210 AD to 500,000 in 273[41] to 35,000 after the Gothic War,[42] reducing the sprawling city to groups of inhabited buildings interspersed among large areas of ruins, vegetation, vineyards and market gardens.[43]

After the Lombard invasion of Italy, the city remained nominally Byzantine, but in reality the popes pursued a policy of equilibrium between the Byzantines, the Franks and the Lombards.[44] In 729, the Lombard king Liutprand donated to the church the north Latium town of Sutri, starting the temporal power of the church.[44] In 756, Pepin the Short, after having defeated the Lombards, gave to the Pope temporal jurisdiction over the Roman Duchy and the Exarchate of Ravenna, thus creating the Papal States.[44] Since this period three powers tried to rule the city: the pope, the nobility, together with the chiefs of militias, the judges, the Senate and the populace; and the Frankish king, as king of the Lombards, patricius and Emperor.[44] These three parties (theocratic, republican and imperial) were a characteristic of Roman life during the entire Middle Ages.[44] On the Christmas night of 800, Charlemagne was crowned in Rome as emperor of the Holy Roman Empire by Pope Leo III: on that occasion the city hosted for the first time the two powers whose struggle for the universal power was to be a constant of the Middle Ages.[44]