Pope Sixtus III was elected as the forty-fourth Pope of the Holy Church on July 31, AD 432. He successfully restored peace within the Church by resolving the discord that arose from the Ephesus Synod. As the Successor of Peter, the Pope diligently worked to preserve the unity of the Church by mending the divisions caused by sectarianism and bringing together the disunited factions.

To resolve the schism resulting from the Council of Ephesus, Pope Sixtus III showed willingness to reconcile with Mar John of Antioch and other excommunicated church leaders. As a condition for union with the Church, the Pope required Mar John of Antioch to accept the teachings of the Ephesian Synod and renounce the teachings of Nestorius.

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While Pope Sixtus III had initially shown favor towards the heresy of Pelagianism, he accepted Pope Zosimus' judgment that it was incorrect. He recognized the importance of Saint Augustine's teachings as a source of inspiration in matters of apostolic learning.

The Pope actively sought reconciliation between the Antiochians, who emphasized Christ's humanity in their teachings, and the Alexandrians, who emphasized Christ’s divinity in their teachings. These two groups had been divided by the Nestorian heresy and the Ephesian Synod. In AD 433, a consensus was reached on the nature of Christ's divinity and humanity, and they signed the Formula of Union, a document defining their shared beliefs.

While Pope Sixtus III maintained positive relations with Rome and the Eastern Church leadership, his efforts faced opposition in AD 434. Proclus, the new Patriarch of Constantinople, attempted to separate the Apostolic Vicariate of Illyricum from Rome and bring it under the control the Eastern Church. In response, Pope Sixtus III ordered the bishops of Illyricum to disregard Proclus' attempts and appointed the bishop of Thessalonica as the Apostolic Vicar of Illyricum. He also urged Proclus not to entertain appeals from the Illyrian bishops to Constantinople.

In AD 410, Pope Sixtus III undertook the restoration of Rome, which had been damaged by the Visigothic invasion. He rebuilt the damaged churches, including the Lateran Baptistery and the Basilica of Saint Mary Major, in their present octagonal shape. Inscriptions in the Lateran Baptistery conveyed the significance of baptism and the necessity of grace for salvation. The frescoes in the Basilica of Saint Mary Major depicted the Church's triumph over Nestorianism and the proceedings of the Synod of Ephesus. Additionally, Pope Sixtus III sought Emperor Valentinian's assistance in replacing the valuable gold and silver ornaments and objects looted from the Basilicas of Peter and Paul and the Lateran Basilica. He also established the first monastery in Rome, dedicated to Saint Sebastian.

After a meaningful and effective reign of approximately eight years, Pope Sixtus III passed away on August 19, AD 440. His physical remains were laid to rest in Saint Lawrence's Cemetery.
-edit&transl. SM