At the council of Arles (314) bishop Theodor [Teodoro] wrote his subscription as de civitate Aquileiensi, provincia Dalmatia, to indicate the authoritativeness of the eastern Adriatic lands he represented. The Aquileian church prestige is especially documented during the bishoprics of Valerianus [Valeriano] (post 345-388) and Chromatius [Cromazio] (388‑408). In the fourth-fifth century the metropolitan authority extended over the province of Venetia et Histria. The existence of the Aquileian metropolitan province is documented with certainty since the first half of the fifth century. Upon pope Leo the Great’s injunction in 442 the metropolitan bishop Ianuarius called up the provincial council of the Churches of Venetia et Histria with an anti-Pelagian purpose. On March 21st, 458 the same Leo the Great communicated the subsequent patriarch Nicetas the provisions he and the suffragan bishops of his province had to take towards their faithful after Attila’s devastation. The borders of the Aquileian jurisdiction in the time of its transfer to Grado are described in three documents: the list of the bishops participating in the provincial councils of Grado (572/77) and Marano (589/90), as well as in the letter the bishops of the metropolis addressed to emperor Mauritius in 591. There is therefore documentary evidence of 22/24 suffragan Episcopal sees: in Venetia et Histria: Asolo, Altino, Belluno, Rovigno, Concordia, Feltre, Zuglio, Oderzo, Parenzo, Padua, Pola, Treviso, Trieste, Trent, Verona, Vicenza and, since the beginning of the seventh century, the diocese of Como that, though being in the Milanese metropolis, adhered to the Schism of the Three Chapters [scisma tricapitolino]; in the Mediterranean Noricum (Carinthia): Aguntum (Lienz), Teurnia (St. Peter im Holz) and Virunum; in Savia (Slovenia): Celeia (Celje), Aemona (Ljubljana); in Pannonia I (southern Hungary): Scarabantia (Sopron); in Retia II (southern Switzerland) Sabiona (Säben) and, probably, Augusta Vindelicorum (Augsburg) in Retia II. Under the urge of Avar and Slavic invasions (sixth-seventh century) some of them were abandoned by their titular bishops and inhabitants and, being devastated, they did no longer arise. As early as the fifth century the Aquileian bishop’s metropolis was therefore to include the territories of Venetia et Histria, Raetia secunda, Mediterranean Noricum, First Pannonia and Savia. This territory was very large and bordered on the west by the river Po in its length between the mouth and the confluence of the Mincio, then northward it included the lake of Garda and got to the Danube at its confluence with the Iller; it then went forth on the north along the Danube to Brigentium; on the east it was bordered by a line that almost straight went down to the south of the Sava, and then parallel to this river went westward so as to reach and include the western part of the Istrian peninsula. In 952 Otto I united the March of Friuli and Istria with the Duchy of Bavaria and Carinthia. There was a number of provincial councils, which took place, beyond Aquileia, also in Cividale and in other cities of the patriarchal metropolis, the last of which was celebrated in October 1596 in the cathedral of Udine, the patriarchs’ last residence. Therein participated the sixteen historical and canonical dioceses that had been suffragan since the thirteenth century: Como, Trento, Verona, Padova, Vicenza, Treviso, Concordia, Ceneda, Feltre, Belluno, Pula, Parenzo, Trieste, Pedena, Iustinopolis (Koper), Emona-Lubljana. The diocese of Mantua was suffragan until 1452.
The deployment of the Episcopal sees of the Aquileian metropolis