ILLIRICO (Illyricum). – Coastal region east of the Adriatic, originally comprising Dalmatia and Albania, which under the Roman Empire progressively extended inland as far as the Danube and along the sea as far as the Aegean, excluding Thrace. It was evangelized by St. Paul (Rom. 15:19), who stayed there several times until the end of his apostolic activity (Tit. 3:12: he goes to Nicopolis, where he summons Titus). Praetorian prefecture from Diocletian onward; in the Constantinian period, when the ecclesiastical hierarchy had not yet been organized, there corresponded to it a group of episcopal sees which, by force of circumstances, were in frequent relations with one another, as appears from the Council of Sardica (343–44); during the same fourth century, however, two sees, imperial residences, began to assume particular importance: Sirmium for western I. and Thessalonica for eastern I.; with the decline of Sirmium, Aquileia, Milan (St. Ambrose), and later Salona gained influence in that area; Thessalonica, from the end of the fourth century, began to receive important commissions from the Pope, so that the institution of the so-called vicariate, almost a form of legation, gradually took shape.
Thus, in the fifth century, a complete hierarchy can be seen, formed of provinces corresponding to the civil provinces: Ripuarian and Mediterranean Noricum (metropolitan sees of Lusitanum and Teurnia), Dalmatia (metropolitan see of Salona), Pannonia I, Valeria, Pannonia II (metropolitan see of Sirmium), and Savia, forming the western group; Moesia I, Ripuarian Dacia, Mediterranean Dacia (metropolitan see of Sardica), Dardania (metropolitan see of Scupi, later Justiniana Prima), Praevalitana (metropolitan see of Scodra), Macedonia (metropolitan see of Thessalonica), New Epirus (metropolitan see of Dyrrachium), Old Epirus (metropolitan see of Nicopolis), Thessaly (metropolitan see of Larissa), Achaea (metropolitan see of Corinth), and Crete, which, however, remained ecclesiastically almost separate.
The rise of Constantinople, as capital of the Empire and therefore also as a highly important episcopal see and subsequently even a patriarchal see, on the one hand, and the many relations of I. with the Roman See, on the other, made this region the classic field of contention between old and new Rome in matters of jurisdiction (schism was not yet being discussed); beginning with St. John Chrysostom, Constantinople made attempts to extend Byzantine influence over I., but because of the country’s Western orientation and papal vigilance, they came to nothing; the ecclesiastical history of Illyricum in the fifth, sixth, and seventh centuries is an entire fabric of papal administrative measures, exercised directly or through the bishop of Salona, for western I. (without any interference by Constantinople in this part); and directly or through the “vicar” of Thessalonica or another delegate, for eastern I.; moreover, the Roman See exercised influence over I. as far as Scodra through the administrators of the papal estates in Dalmatia and Praevalitana. Under Justinian (535), Thessalonica, which had by then practically lost the vicariate because of its participation in the Acacian Schism (end of the fifth century; beginning of the sixth), lost it in favor of the Emperor’s homeland, the newly established Justiniana Prima, and of other sees that from time to time received important commissions. The result was that western I. and the northern provinces of eastern I. (especially Praevalitana) always maintained a distinctly Roman character and orthodoxy, while the southern ones, even if they sometimes allowed themselves to be drawn into heretical or schismatic movements, always ended by returning to rectify their position.
The constant position of the Illyrian bishops at the great Councils (Ephesus, Chalcedon, etc.) is characteristic, for example: although they included representatives of considerable theological ability, they remained hesitant or neutral at first, until, as soon as the Roman position had crystallized, they automatically reattached themselves to II.
The barbarian invasions that submerged most of the peninsula caused, on the one hand, almost all the ancient sees to disappear, and, on the other, made relations between Rome and the surviving sees difficult; the iconoclast Leo the Isaurian took advantage of this to detach I. from Rome and assign it to Constantinople. The Adriatic coast from Scodra northward, however, continued in its tradition, whereas from that time the rest of the region, despite papal protests and local resistance, was in practice lost to direct Roman jurisdiction; the final act of the drama concerned jurisdiction over the Bulgars (second half of the ninth century), who had settled in Illyrian provinces and were therefore claimed by Rome; from that time, also in matters of heresy and schism, eastern I. would, if not always, very often follow Constantinople; it is true, however, that by then the ecclesiastical toponym “I.” was no longer officially used.