ILLIRICO (Illyricum). – A coastal region to the east of the Adriatic, originally comprising Dalmatia and Albania, which under the Roman Empire progressively extended inland as far as the Danube and on the sea as far as the Aegean, excluding Thrace. It was evangelized by St. Paul (Rom 15:19), who visited it several times until the end of his apostolic activity (Tit 3:12: he goes to Nicopolis, where he calls Titus). From Diocletian onwards, it was a praetorian prefecture; in the Constantinian era, when the ecclesiastical hierarchy was not yet organized, a group of episcopal sees emerged through force of circumstances, maintaining frequent relations with one another, as appears from the Council of Serdica (343–44). However, during the same 4th century, two sees began to gain special importance as imperial residences: Sirmium for western Illyricum and Thessalonica for the eastern part. With the decline of Sirmium, Aquileia, Milan (St. Ambrose), and later Salona acquired influence in that area; Thessalonica, from the end of the 4th century, began to receive important assignments from the Pope, so that the institution of the so-called vicariate, almost a kind of legation, began to take shape.
Thus, in the 5th century, a complete hierarchy is seen, formed by provinces corresponding to the civil provinces: Noricum Ripense and Mediterraneum (metropolises of Laureacum and Teurnia), Dalmatia (metropolis of Salona), Pannonia I, Valeria, Pannonia II (metropolis of Sirmium), Savia, forming the western group; Moesia I, Dacia Ripensis, Dacia Mediterranea (metropolis of Serdica), Dardania (metropolis of Scupi), then Justiniana Prima, Praevalitana (metropolis of Scodra), Macedonia (metropolis of Thessalonica), Epirus Nova (metropolis of Dyrrachium), Epirus Vetus (metropolis of Nicopolis), Thessaly (metropolis of Larissa), Achaea (metropolis of Corinth), and Crete, which, however, remained almost independent ecclesiastically.
The rise of Constantinople as the capital of the Empire and thus as an extremely important episcopal and later patriarchal see, on the one hand, and the many relations of Illyricum with the Roman See, on the other, made this region the classic battleground between Old and New Rome in matters of jurisdiction (though the term "schism" was not yet used). Attempts were made, beginning with St. John Chrysostom, by Constantinople to extend Byzantine influence over Illyricum, but these failed due to the western orientation of the country and the vigilance of the papacy. The ecclesiastical history of Illyricum in the 5th, 6th, and 8th centuries is woven with papal administrative measures, exercised either directly or through the bishop of Salona for western Illyricum (with no interference from Constantinople in this part), and directly or through the "vicar" of Thessalonica or another appointee for the eastern part. Moreover, the Roman See exercised influence over Illyricum as far as Scodra through the administrators of the papal patrimonies in Dalmatia and Praevalitana. Under Justinian (535), Thessalonica, which had practically lost its vicariate due to its participation in the Acacian schism (end of the 5th century; beginning of the 6th), lost it in favor of the Emperor’s homeland, the newly created Prima Justiniana, and other sees that at times received important assignments. The result was that western Illyricum and the northern provinces of the eastern part (especially Praevalitana) maintained a distinctly Roman character and orthodoxy, while the southern provinces, though sometimes drawn into heretical or schismatic movements, always ended up correcting their position.
A characteristic example is the consistent stance of the Illyrian bishops at the great Councils (Ephesus, Chalcedon, etc.): despite having representatives of strong theological value, they initially remained hesitant or neutral, but as soon as the Roman position became clear, they automatically aligned themselves with II.
The barbarian invasions, which submerged most of the peninsula, led on the one hand to the almost complete disappearance of the ancient sees, and on the other to the difficulty of relations between Rome and the surviving ones. The iconoclast Leo III the Isaurian took advantage of this to detach Illyricum from Rome and assign it to Constantinople: the Adriatic coast from Scodra upwards, however, continued in its tradition, while from then on the rest of the region, despite papal protests and local resistance, was effectively lost to direct Roman jurisdiction. The final act of this drama unfolded over the jurisdiction of the Bulgarians (second half of the 9th century), who had settled in Illyrian provinces and were claimed by Rome; from then on, even in matters of heresy and schism, if not always, very often the eastern Illyricum followed Constantinople. It is true, however, that by this time the ecclesiastical toponym "Illyricum" was no longer officially used.