When Christianity became the official religion of the Roman Empire, Pope Leo I began using the title Pontifex Maximus around 440 to emphasize the authority of the Pope.
Victor of Tonnena agrees, adding the detail that Pope Leo I negotiated with Geiseric for the security of the city's population.
From the days of Pope Leo I (440–461) the pontifical household had included papal chamberlains who were personal attendants on the Pope in his private apartments.
Pope Leo the Great is quoted pronouncing, “Let the one who is going to rule over all be elected by all,”.
In 445, Leo disputed with Patriarch Dioscorus, St. Cyril's successor as Patriarch of Alexandria, insisting that the ecclesiastical practice of his see should follow that of Rome on the basis that Mark the Evangelist, the disciple of Saint Peter and founder of the Alexandrian Church, could have had no other tradition than that of the prince of the apostles.
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Besides the above works and a commentary against various sects of Monophysites (Severians, Theodosians, Cainites and Acephali) he left eleven discourses in defence of Pope Leo I and the Council of Chalcedon, also a work against the Agnoetae, submitted by him before publication to Pope Gregory I, who after some observations authorized it unchanged.
This judgment is the more interesting as being in distinct conflict with the opinion of the bishop of Rome—Leo—who, departing from the policy of his predecessor Celestine, had written very strongly to Flavian in support of the doctrine of the two natures and one person.
Pope Leo I had regulated the boundaries of the ecclesiastical provinces of Arles and Vienne: under the latter he left the Dioceses of Valence, Tarentaise, Geneva and Grenoble, but all the other dioceses in this district were made subordinate to Arles.
In Rome on the other hand, the pope appears with ever-increasing frequency as the advocate of the needy population; thus Pope Leo I intercedes with Attila the Hun king and Geiserich the Vandal king, and Pope Gelasius I with Theodoric the Ostrogothic king.