Saint Hilarius succeeded Pope Leo the Great on 19 November AD 461. The reign of Pope Hilarius was peaceful and free of major disturbances. Hilarius, who was the archdeacon of Pope Leo, was one of the Pope's representatives sent to the Synod of Ephesus in AD 449.

Dysorius' partisans threatened and attempted to attack them because they supported Flavian, the Patriarch of Constantinople, and refused to accept the council's decisions. However, Hilarius and his party miraculously escaped from Ephesus.

Pope Hilarius strove to follow in the footsteps of his predecessor and maintain his policies. Like Pope Leo, he did not interfere in the administrative affairs of the Eastern Church. However, he reaffirmed Pope Leo's famous official document 'Thomas ad Flavianum', which refuted many of the heresies that had arisen in the Church. He emphasized the importance of adhering to the canons and canonical studies of the universal canons of Nicaea (325), the Ephesian canon (431), and the Chalcedonian canon (451). Based on these principles, Pope Hilarius also issued a decree to the bishops of the Eastern Church.

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To prevent the heresy of Arianism, which denied the divinity of Christ, Pope Hilarius took precautions and strongly opposed its supporters. He compelled Emperor Anthemius to pledge that he would use all means to prevent heretics from meeting in Rome.

Pope Hilarius was also involved in ecclesiastical affairs in Gaul and Spain. He wrote letters to the bishops and religious communities in Spain, emphasizing their dependence on Rome for church matters. These letters also indicated his exercise of pastoral authority over them as the successor of Peter. In AD 465, he convened a council in Rome, which took place in the Basilica of Mary Major. The council issued a decree stating that bishops should not appoint successors on their deathbeds.

Pope Hilarius had a strong belief that John the Baptist intervened in his miraculous escape from Ephesus. He sought refuge in the tomb of John the Baptist outside the city of Ephesus. After his election as Pope, he built three chapels adjoining the Lateran Basilica in Rome, with one of them being dedicated to the name of John the Baptist.

Furthermore, Pope Hilarius presented valuable artifacts to the people as compensation for the artifacts that had been looted during the Vandal revolt in AD 455. On February 29, AD 468, Hilarius peacefully passed away. He was laid to rest in the monastery of Lawrence, which he had established.
-edit&transl. SM