Though the beginnings of Aquileian Christendom, which should date back to the preaching of St. Mark and his disciple Hermagoras according to tradition, are not exactly known, the organization of the Church in Aquileia was definitely set up around the half of the third century. The number of historically documented martyrs (Felix and Fortunatus, the two Cantiani brothers and their sister, Protus, their pedagogue, Chrisogonus, bishop) is relatively large. The diocese territory first began to take its shape within the precincts of the municipium of Aquileia. Since the end of the fourth century the diocese was bordered by the diocese of Concordia on the west of Tagliamento river; by Iulium Carnicum (Zuglio) on the north; by Aemona (Ljubljana) on north-east; by Parentium (Parenzo/Poreč) in Istria on south-east, and then by Triest after the establishment of the new diocese of Tergeste (Triest) between Aquileia and Parenzo during the Justinian’s Age imperial reformation. After the Edict of Milan (313) the events contradistinguishing the history of the diocese were the construction of the cathedral complex, starting from the Aulae of the bishop Theodor (314 ca) and the blossoming of Christian and monastic life during the bishoprics of Fortunatianus [Fortunaziano] (343 ca – 368 ca), Valerianus [Valeriano] (368 ca – 388) and Chromatius [Cromazio] (388 – 407/408), followed by the dramatic upsetting involving the destruction of Aquileia by Attila (451), the Schism of the three Chapters [scisma dei Tre Capitoli], which broke the union of the Church of Aquileia with the Church of Rome (557 – 698), and the Lombard invasion (568). According to Paul the Deacon’s [Paolo Diacono] telling, when Alboin with his army crossed over the Alps in 568, patriarch Paul I [Paolo I], dreading his ferocity, escaped from Aquileia to the island of Grado and brought the treasure of his Church with himself: books and relics of the martyrs. In Grado he was buried the subsequent year, like his successor Probinus and Elijah [Elia], during whose bishopric the new cathedral was consecrated in the island. Around the year 606 the election of two different patriarchs, which occurred due to the so-called Schism of the three Chapters, gave start to a double series of bishops: the ones of Grado in the Byzantine area, who decided to get united again with Rome under the political and military pressure of the exarch of Ravenna, and the ones of Aquileia in the Lombard territory, who relied on the support of the Lombard kings and persisted in the schism. Interlaced with the territorial division, as it turned out to be in the period between the late Antiquity and the beginning of the Middle Ages, the events of the Schism of the three Chapters [scisma tricapitolino] in an irreversible manner determined the geographical configuration and historical developments of the Aquileian diocese in the following millennium. When pope Serge I put an end to the schism upon an agreement with the Lombard kingdom in 698, he accepted that the ecclesiastical border between the two patriarchies coincided with the political border between the territories under the imperial rule and the areas of the Lombard domain. As to the geographical extent, since the beginning of the seventh century the Aquileian diocese had incorporated the territories of Iulium Carnicum (Zuglio) and the two dioceses in Savia (Carniola), Celeia (Celje) and Aemona (Ljubljana) (which would be re-established only in 1461), which had been devastated by the Lombard, Slavic and Avar invasions. With the reorganization of the dioceses of the empire established by Charlemagne in 811 the Drave river was chosen as a border between Aquileia and Salzburg: that also brought to the definition of the territories of competence for the missionary action of the two Churches towards the East.
The Church of Aquileia since its origins