That meeting dealt primarily with Christology and elucidated the definition of Christ's being as the hypostatic union of two natures, divine and human, united in one person, "with neither confusion nor division".

About this time Cyril of Alexandria appealed to Rome regarding a jurisdictional dispute with Juvenal of Jerusalem, but it is not entirely clear whether the letter was intended for Leo in his capacity as archdeacon,[5] or for Pope Celestine I directly.

[7] In late 443, Leo preached a series of sermons condemning the Manichaeans and calling for Romans to denounce suspected heretics to their priests.

Bishop Turibius of Astorga, astonished at the spread of the sect in Spain, had addressed the other Spanish bishops on the subject, sending a copy of his letter to Leo, who took the opportunity to write an extended treatise (21 July 447) against the sect, examining its false teaching in detail and calling for a Spanish general council to investigate whether it had any adherents in the episcopate.

The Pope justified the death penalty declaring that if the followers of a heresy were allowed to live, that would be the end of human and Divine law.

[16] Faced with this decree, Hilary submitted to the pope, although under his successor, Ravennius, Leo divided the metropolitan rights between Arles and Vienne (450).

[17] The fact that the African province of Mauretania Caesariensis had been preserved to the empire and thus to the Nicene faith during the Vandal invasion and, in its isolation, was disposed to rest on outside support, gave Leo an opportunity to assert his authority there.

Pope Innocent I had constituted the metropolitan of Thessalonica his vicar, in order to oppose the growing influence of the patriarch of Constantinople in the area.

In a letter of about 446 to a successor bishop of Thessalonica, Anastasius, Leo reproached him for the way he had treated one of the metropolitan bishops subject to him; after giving various instructions about the functions entrusted to Anastasius and stressing that certain powers were reserved to the pope himself, Leo wrote: "The care of the universal Church should converge towards Peter's one seat, and nothing anywhere should be separated from its Head.

At the Second Council of Ephesus in 449, Leo's representatives delivered his famous Tome,[19] a statement of the faith of the Roman Church in the form of a letter addressed to Archbishop Flavian of Constantinople, which repeats, in close adherence to Augustine of Hippo, the formulas of western Christology.

The council did not read the letter nor did it pay any attention to the protests of Leo's legates but deposed Flavian and Eusebius of Dorylaeum, who appealed to Rome.

"[21][22][23]Leo firmly declined to confirm their disciplinary arrangements, which seemed to allow Constantinople a practically equal authority with Rome and regarded the civil importance of a city as a determining factor in its ecclesiastical position; but he strongly supported its dogmatic decrees, especially when, after the accession of Emperor Leo I (457), there seemed to be a disposition toward compromise with the Eutychians.

Subsequently, through numerous letters to bishops and members of the imperial family, Leo incessantly worked for the propagation and universal reception of the faith in Christ as defined by Chalcedon, also in the eastern part of the Roman empire.

[26] Ephesus III accused Leo's formula of two natures after the union as being fundamentally no different to the view of Nestorius, contradicting Cyril of Alexandria's formula of "mia physis tou Theo logou sesarkōmenē", or "one (mia) nature of the Word of God incarnate" (μία φύσις τοῦ θεοῦ λόγου σεσαρκωμένη).

The Coptic Orthodox Christians are Miaphysites, which means they believe that Jesus Christ is both 100% human and 100% divine but in one person without mingling, confusion or alteration.

In every liturgy till this day, the Copts recite “Christ's divinity parted not from His humanity, not for a single moment nor a twinkling of an eye".

[30] Leo assumed the papacy at a time of increasing barbarian invasions; this, coupled with the decreasing imperial authority in the West, forced the Bishop of Rome to take a more active part in civil and political affairs.

He was one of the first bishops of Rome to promote papal primacy based on succession from Peter the Apostle; and he did so as a means of maintaining unity among the churches.

[citation needed] After the indecisive outcome of the Battle of Chalons in 451, Attila invaded Italy in 452, sacking cities such as Aquileia and heading for Rome.

In response, the emperor sent three envoys to negotiate with Attila: Gennadius Avienus, one of the consuls of 450, Memmius Aemilius Trygetius, the former urban prefect, and Leo.

[32] Another near-contemporary was the historian Priscus who records that Attila was dissuaded from attacking Rome by his own men because they feared he would share the fate of the Visigothic king Alaric, who died shortly after sacking the city in 410.

A trustworthy chronicle hands down another account which does not conflict with the fact that an embassy was sent, but evidently furnishes the true reasons which moved Attila to receive it favourably.

Plague broke out in the barbarian host and their food ran short, and at the same time troops arrived from the east, sent by Marcian to the aid of Italy.

The question of Honoria was left unsettled, and he threatened that he would come again and do worse things in Italy unless she were given up with the due portion of the Imperial possessions.

[35]Leo's intercession could not prevent the sack of the city by the Vandal King Genseric in 455, but murder and arson were repressed by his influence.

Let the gentile take courage in that he is called to life... Let us put off then the old man with his deeds: and having obtained a share in the birth of Christ let us renounce the works of the flesh.

Christian, acknowledge thy dignity, and becoming a partner in the Divine nature, refuse to return to the old baseness by degenerate conduct.

The relocation was apparently due to the number of later papal burials obscuring the prominence that Sergius believed Leo's tomb should have.