Hilarius came from Sardinia and was pope for seven years. He took office after Leo I and it was in his spirit that his pontificate passed. He was an ardent opponent of Arianism and all kinds of deviations which questioned the dogma of divine and human nature of Christ, agreed on at the Council of Nice in 325 A.D., and which still caused heated discussions across three continents. However, despite his hostile view towards dissenters the pope was forced to compromise, which can be attested to by the construction of the Church of Sant’Agata dei Goti (approx. 470 A.D.) in Rome, destined for the Arian mercenaries residing in Rome. Hilarius devoted special attention to St. John the Evangelist, looking for his intercession at a critical moment of his life, when during the Council of Ephesus in 449 A.D. (the so-called robber council), while still a papal legate he barely escaped with his life during heated theological disputes, which all parties decided to solve with arms. In thanks for protection, he build a small chapel for the saint, which was adjacent to the San Giovanni in Laterano Baptistery). Two other chapels also found in the church, were also a result of his patronage. Today his donation is attested to by an original inscription found on the lintel of the entrance to the chapel of St. John, informing that the pope was saved thanks to the aid of this evangelist.

Another building which was supervised and to a large extent financed by the pope was the Oratory of St. Lawrence in the complex of the nearly ruined cemetery Basilica of San Lorenzo fuori la mura, in which two libraries were also constructed (a Greek one and a Latin one), a bath, as well as a shelter for pilgrims, and finally a monastery of St. Stephen.

According to Liber Pontificalis, the pope was buried in direct proximity to St. Lawrence. Many centuries later, in 1654, the earthly remains of Hilarius (who was by that time a saint) were moved, or rather presented to various church dignitaries, until finally they came into the possession of Mario Albricci, a cardinal and an owner of the town of Mesagne, most likely in order to raise the status and prestige of the cardinal’s seat. Until the present day they are still found in the town’s Basilica of Vergine Samtissima del Carmelo.

Structures build in Rome during the pontificate of Pope Hilarius:

• Three chapels (oratories) added to the Baptistery of St. John in Lateran.
• Chapel of the Holy Cross (Santa Croce) devoted to storing relics of the Holy Cross, adorned with mosaics in parts of the ceiling and in parts near the windows and with marble slabs on the walls (opus sectile), torn down in the XVI century. The chroniclers also spoke of the figures of SS. Peter, Paul and both the Saint Johns near the windows as well as angels carrying the cross on the ceiling.
• Chapel of St. John the Baptist (preserved), adorned with mosaics just as the previous one had been – thoroughly modernized in the XVIII century
• Chapel of St. John the Evangelist (preserved) adorned with mosaics from the V century on the ceiling (The Lamb of God in the central part)
• Church of San Stefano Rotondo – the commencement of the construction of this significant structure is attributed to by some researchers the end of the pontificate of Hilarius
• At the initiative of Hilarius, in the then Church of Sant’Anastasia (present day church at the foot of Palatine Hill) a mosaic decoration was completed in the apse (non-existent today)