Saint Leo I was one of the most powerful and greatest popes in the history of the Holy Church to succeed Peter’s throne. Despite being only a deacon, he was on a diplomatic mission in Gaul as the emperor's representative when he was elected pope. After completing his mission, Pope Leo returned to Rome and on September 29, AD 440, he was consecrated bishop of Rome.
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The reign of Pope Leo marked a turning point in the history of the Church and the papacy. As Bishop of Rome and Peter's successor, Pope Leo unambiguously declared universal and supreme authority for the popes in the Catholic Church. Although previous popes had often asserted their right to succession based on Peter's throne, it was Pope Leo who first used the title "successor of Peter." In his homily on the fifth anniversary of his election, he proclaimed, the Pope "transmits to his successors the assurance and stability which Peter received from Christ." Since then, all the popes elected as successors of Pope Leo have exercised their pastoral authority over the faithful and bishops of the Church as the representatives of Peter, the patriarch of the Church.
As Peter's successor, Pope Leo exercised his pastoral authority over bishops and dioceses throughout Italy, including Milan and the northern provinces, and imposed strict regulations on their affairs. He initiated efforts to establish pastoral unity throughout Italy, rectify vices and misconduct, and resolve disputes and quarrels. When the bishops of Spain sought Pope Leo's support in combating the heresy of Priscillianism, he provided the necessary instructions to combat this heresy.
Although African ecclesiastics upheld their pastoral autonomy and were cautious about Rome's interference, they accepted and wholeheartedly embraced papal decrees concerning irregularities in the election of church leaders and local conflicts. Although Pope Boniface later revoked the authority granted by Pope Zosimus over the Diocese of Arles and the Bishops of Arles and the dioceses and congregations of the province of Gaul, Pope Leo strongly urged Bishop Hilary of Arles to confine his pastoral authority and missions within the boundaries of his diocese. Bishop Hilary was instructed not to interfere in the selection and appointment of bishops in other dioceses. Pope Leo ordered that the bishop of a diocese should be elected by the priestly community and the spiritual leaders of that diocese, with formal approval from the faithful community. This principle established by Pope Leo regarding the election of bishops continues to be followed in the Church today as a normative principle.
Four days after Pope Leo's decision, Valentinian III, the ruler of the Western Empire, ordered his generals to immediately carry out the papal order in Gaul. The emperor justified his acceptance of papal authority by citing the importance of the city of Rome. However, this caused discontent among the Pope, as Rome's political significance had greatly declined by that time. As a result, the political and ecclesiastical leadership of the Eastern Empire actively worked to shift the center of church administration from Rome to Constantinople. The Eastern church leadership was not fully willing to accept Pope Leo's apostolic claims like the Western church community.
The study of Monophysitism was a contentious topic of theological debate in the Church, particularly in the Eastern Church. The monk Eutychius, an abbot of Constantinople, rejected the teaching that Christ had the same human nature as other men. Eutychus taught that at the time of the Incarnation, Christ's human nature and divine nature were united, resulting in Christ having only one nature, the divine nature. However, Flavian, the bishop of Constantinople, condemned Eutychus' teaching as heretical. In AD 448, a council convened in Constantinople declared Eutychius' teaching false and adopted Cyril of Alexandria's doctrine of Dyophysitism. Dyophysitism holds that even after the human incarnation of Christ, His human nature and divine nature remain united. Subsequently, Eutychus appealed to Pope Leo, while Bishop Flavian also informed the Pope about the Council's decision. On June 13, AD 449, Pope Leo responded with the famous decree known as "Tomus ad Flavianum," addressed to Bishop Flavian. This decree clarified the Church's teaching on Christ, stating unequivocally that in Christ, two natures—the human nature and the divine nature—are united as one.
Meanwhile, in an attempt to resolve these issues, Emperor Theodosius II convened a new Synod in Ephesus on August 1, 449. Pope Leo sent three of his representatives to Ephesus, carrying the decree "Tomus ad Flavianum," which contained the orthodox teachings of the Church. Pope Leo I hoped that the synod fathers would accept his teaching in the decree, but the synod, led by Dioscorus of Alexandria, disregarded the Pope's teachings. Due to Dioscorus' opposition to Flavian and the Antiochians, Eutychus, who had been condemned by Flavian for his Monophysite studies, was acquitted, and Flavian was exiled, despite strong disapproval from the papal legates. Flavian died on his way to exile. The Emperor enforced the decisions of the synod through coercion and threats. Recognizing the deteriorating situation, the papal delegation escaped and returned to Rome to inform the Pope. Pope Leo I refused to accept the synod or its decisions, describing it as the "council of robbers" ("Latrocinium").
Subsequently, Pope Leo I requested Emperor Theodosius II to convene a new synod, but the Emperor rejected the Pope's demand. However, following the unexpected death of Emperor Theodosius on July 28, AD 450, his sister Pulcheria, a devout believer in the teachings of the Church, assumed power and married the orthodox and devout Marcian, who became the Emperor. Pope Leo accepted Pope Marcian's request to convene another synod and agreed to preside over it through his representatives. Thus, on October 8, 451, a new synod was held at Chalcedon. Pope Leo proposed to the Emperor that he himself would preside over the synod through his representatives, and the Emperor agreed. This marked the first time in the history of the Holy Church that an ecumenical synod was convened under the presidency of the Bishop of Rome.
Pope Leo sent two bishops and two priests as his representatives to the Synod of Chalcedon. Bishop Paschasius presided over the council in the Pope's name. The synod found Dioscorus guilty and banished him. Additionally, under the threat of death, it sympathized with the bishops who had been forced to sign the decisions of the Council of Ephesus in 449. The synod repudiated the teachings of the Council of Ephesus and accepted Pope Leo's teachings, which affirmed that in Christ, two natures—the human nature and the divine nature—are united. Proclaiming that "Peter has spoken through Leo," the Synod accepted Pope Leo's "Tomus ad Flavianum" as the official teaching of the Church.
On March 21, 453, the Council of Chalcedon adopted all the teachings of the Synod, except for Canon 28. Pope Leo did not accept the 28th canon, which placed Constantinople second only to Rome as a royal city, equal to Rome in the Eastern Empire, and designated the Patriarch of Constantinople as the Universal Patriarch. During Pope Leo's subsequent reign, Julian, Bishop of Cos, represented him at the imperial court in Constantinople.
In addition to successfully preserving the Church's faith through his bold and faithful teachings, Pope Leo also managed to safeguard Rome from attacks by Germanic tribes through his diplomatic skills. In AD452, when the Huns, led by their King Attila, entered Italy to attack the city of Rome, Pope Leo met him at Mantua with a royal delegation and dissuaded him from proceeding. Similarly, in AD455, when the Vandals, under their king Gaiseric, approached the city walls of Rome to attack, the Pope attempted to dissuade them, although Gaiseric refused to relent. However, he refrained from destroying the city and committing massacres, but his troops plundered Rome for fourteen days.
Pope Leo I, who courageously led the Church in faith for twenty years, passed away on November 10, AD461. He was buried in St. Peter's Basilica. In 1751, Pope Benedict XV canonized Pope Leo.
-edit&transl. SM