Suffragan of the patriarchate of Aquileia

The Patriarchate of Aquileia was an episcopal see in northeastern Italy, centred on the ancient city of Aquileia situated at the head of the Adriatic, on what is now the Italian seacoast. For many centuries it played an important part in history, particularly in that of the Holy See and northern Italy, and a number of church councils were held there.

No longer a residential bishopric, it is today classified as a titular see.

Ancient tradition asserts that the see was founded by St. Mark, sent there by St. Peter, previous to his mission to Alexandria. St. Hermagoras is said to have been its first bishop and to have died a martyr's death (c. 70). At the end of the third century (285) another martyr, St. Helarus (or St. Hilarius) was bishop of Aquileia.

In the course of the fourth century the city was the chief ecclesiastical centre for the region about the head of the Adriatic, Regio X of the Roman emperor Augustus' eleven regions of Italy, "Venetia et Histria." In 381, St. Valerian appears as Metropolitan bishop of the churches in this territory; his synod of that year, held against the Arians, was attended by 32 (or 24) bishops. Valerian was succeeded by St. Chromatius (388–408) known for his homiletic and exegetical works. He promoted the work of Sts. Jerome and Rufinus and kept contact with Sts. Ambrose of Milan and John Chrysostom.

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