Upon the demise of Pope Boniface I, the companion of Saint Ambrose, Celestine was elected as the forty-third Pope on September 10, AD 422.

False teachings and movements based upon them gained notoriety in the Fifth Century. As the 43rd Pope, Saint Celestine I battled against them, earning the title “The Heresy Fighter.”

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Immediately after his election, he took control over the churches in Rome who followed the heresy of Novationism. He also commenced restoration work of the Julian Basilica (St. Mary's Basilica) that had been destroyed during the invasion of the Gothic tribe in AD410.

The African Congregation, in a congratulatory letter to Pope Celestine, urged him to uphold the excommunication of Antonius, Bishop of Fussala, who had been deemed unfit by the congregation.

Similar to his predecessor, Saint Zosimus, Pope Celestine found himself involved in unwarranted disputes with the bishops in North Africa. He ordered for the second time the restoration of Apiarius to the priesthood, even though Apiarius had been deposed by his bishop. Apiarius admitted his error at a council in Carthage in AD426. The African church, once again, refused to give in to the Papal decision.

As the Bishop of Rome and the Vicar of Christ, Pope Celestine declared and reminded that he held pastoral authority over the universal Church beyond Rome, encompassing both the Western and Eastern Churches. He reminded the bishops of Gaul that they were under his jurisdiction and expressed disapproval and opposition to various practices deviating from the norm, such as bishops wearing different vestments. With Pelagianism vanquished, Pope Celestine warned against the heresy of Semi-Pelagianism infiltrating the Church.

Semi-Pelagianism, a heretical teaching propagated by Saint John Cassian, posited that the only human effort was required to turn to God, and that God's grace was essential for salvation. The leaders of the community adhering to the heretical teachings of Pelagianism were expelled from Rome and the Western Church. Pope Celestine I dispatched a delegation to Rome with the aim of curbing the growth of Pelagianism. In AD 431, the first missionary group was sent to Ireland, led by Palladius, who was appointed as bishop of Ireland. Due to the local opposition and hostility, Palladius swiftly returned to Britain. Later, Saint Patrick was sent to Ireland and succeeded in converting the island to the faith.

One of the significant developments during Pope Celestine's pontificate was the convening of the third universal Synod in Church history, known as the Synod of Ephesus. Nestorius, the Patriarch of Constantinople, espoused the teachings of the Antiochian School, which emphasized the humanity of Christ. However, their interpretation began to deviate from the teachings and faith of the Church, as they posited a separation between Christ's human and divine natures. Nestorius taught that there were two distinct persons in Christ—the human person and the divine person—and that Mary should be addressed as the Mother of Christ (Christokos) rather than the Mother of God (Theotokos). This sparked a significant controversy within the Church.

Bishop Cyril of Alexandria, a fervent opponent of Nestorius, refuted his teachings and proclaimed that in Christ, humanity and divinity were united, thus justifying the title of Mary as the Mother of God. When Cyril and Nesthorius presented their arguments to Pope Celestine, the Pontiff accepted Saint Cyril's studies. In August AD430, a synod convened in Rome, condemned Nestorius and his teachings as false. The synod demanded that Nestorius recant his teachings within ten days; otherwise, he would face excommunication.

The responsibility for implementing the Pope's decision and the synod's ruling fell upon Cyril of Alexandria. Cyril was instructed by Pope Celestine to approve and sign the Twelve Statements, which Nestorius refused. Instead, Nestorius appealed to the Byzantine Emperor Theodosius II to convene a synod to address the issue.

In an attempt to resolve the matter, Emperor Theodosius II called for a general synod at Ephesus, which commenced on June 7, AD431. The synod began before the arrival of the Antiochian bishops and papal representatives, who later excommunicated Nestorius in their absence.

On July 10, the papal representatives joined the conference and endorsed the synodal decision. Meanwhile, Mar John, the Bishop of Antioch, and his assistants arrived at Ephesus, refusing to accept the decisions of the synod and abstaining from subsequent meetings. This action led Saint Cyril and the other council fathers to depose Mar John and his assistants, resulting in a schism between Antioch and Alexandria. However, the Pope hoped that Mar John and the others would eventually reject Nestorius' teachings, embrace the synodal teachings, and reunite with the Church. As anticipated, the Antiochian community later reconciled and reestablished unity with the Church.

Pope Celestine I passed away on July 27, 432, and was laid to rest in the Priscilla Cemetery along the Via Salaria. His tomb was adorned with depictions of the Council of Ephesus, commemorating his significant involvement in the synodal proceedings.
-edit&transl. SM