Ancient city of Pannonia on the River Sava, the site of modern Hovatzka Mitrovitza in Bosnia. The city was conquered by the Romans under Caecina Severus in the 1st century a.d. and became the capital of Pannonia II under Diocletian. Its early Christian colony included the four crowned martyrs, Pollius the lector, and Irenaeus of Sirmium (d. 309). In the 4th century it served as the metropolitan see for western Illyricum. In the Arian quarrel its bishop, Photinus (d. 343), made himself the champion of the radical Arianism of Eunomius of Cyzicus (Anomoeanism): "The Word is dissimilar to the Father." This doctrine was condemned, along with Photinus, at Antioch (344), Milan (345), and Sirmium (348,351); the last-named synod deposed Photinus. His successor, Germinius, held a synod (summer of 357) and with Valens of Mursa and Ursacius of Singidunum adopted the "Second Formula of Sirmium," which held that the Son was inferior and subordinated to the Father, that the Holy Spirit existed through the Son, a statement that Hosius of Córdoba signed, and that became for a time the official doctrine of Imperial orthodoxy. It was condemned by the Emperor Gratian (378), by Pope damasus i (375), and by the Synod of Aquileia (381).
Destroyed by the Avars (582), the city was rebuilt around the Oriental monastery of St. Demetrius and was called Dmitrovica, while Sirmium (Croatian Sriem) became the name of the region between the Sava and the Danube Rivers. Pope adrian ii in 869 attempted to form an archdiocese of Sirmium with jurisdiction over the central Danubian area, but political intrigue frustrated the project. In 1229 gregory xi made it a diocese, with the former Benedictine monastery at Bonostar as its seat. It was troubled with heretical movements from the socalled Bosnian heresy of the patarines to Calvinism, and counted many apostates to Islam.
Bibliography: w. koch, Lexikon für Theologie und Kirche, ed. m. buchberger, 10 v. (Freiburg 1930–38) 9:597–598. d. farlati, Illyricum sacrum, 8 v., v. 5–8 ed. g. coleti (Venice 1751–1819) 7:449–571. j. zeiller, Les Origines chrétiennes dans les provinces danubiennes (Paris 1918).