Christianity spread to the Balkans beginning in the 1st century. Florus and Laurus are venerated as Christian martyrs of the 2nd century; they were murdered along with 300 Christians in Lipljan. Constantine the Great (306–337), born in Niš, was the first Christian Roman emperor. Several bishops seated in what is today Serbia participated in the First Council of Nicaea (325), such as Ursacius of Singidunum. In 380, Eastern Roman Emperor Theodosius I decreed that his subjects would be Christians according to the Council of Nicea formula. Greek was used in the Byzantine church, while the Roman church used Latin. With the definite split in 395, the line in Europe ran south along the Drina river. Among old Christian heritage is the Archbishopric of Justiniana Prima, established in 535, which had jurisdiction over the whole of present-day Serbia. However, the Archbishopric did not last, as the Slavs and Avars destroyed the region sometime after 602 when the last mention is made of it. In 731 Leo III attached Illyricum and Southern Italy (Sicily and Calabria) to Patriarch Anastasius of Constantinople, transferring the papal authority to the Eastern Church.
Christianization of Serbs
Seal of the prince Strojimir
, from the late 9th century – the oldest artifact on the Christianization of the Serbs
The history of the early medieval Serbian Principality is recorded in the work De Administrando Imperio (DAI), compiled by the Byzantine Emperor Constantine VII Porphyrogenitus (r. 913–959). The DAI drew information on the Serbs from, among others, a Serbian source. The Serbs were said to have received the protection of Emperor Heraclius (r. 610–641), and Porphyrogenitus stressed that the Serbs had always been under Imperial rule. His account on the first Christianization of the Serbs can be dated to 632–638; this might have been Porphyrogenitus' construction, or may have really taken place, encompassing a limited group of chiefs and then very poorly received by the wider layers of the tribe. The establishment of Christianity as state religion dates to the time of Prince Mutimir (r. 851–891) and Byzantine Emperor Basil I (r. 867–886); Porphyrogenitus attests that Croats and Serbs sent delegates asking for baptism, thus Basil "baptized all of them that were unbaptized of the aforesaid nations". The Christianization was due partly to Byzantine and subsequent Bulgarian influence. At least during the rule of Kocel (861–874) in Pannonia, communications between Serbia and Great Moravia, where Methodius was active, must have been possible. This fact, the pope was presumably aware of, when planning Methodius' diocese as well as that of the Dalmatian coast, which was in Byzantine hands as far north as Split. There is a possibility that some Cyrillomethodian pupils reached Serbia in the 870s, perhaps even sent by Methodius himself. Serbia was accounted Christian as of about 870.
The first Serbian bishopric was founded at Ras, near modern Novi Pazar on the Ibar river. According to Vlasto, the initial affiliation is uncertain; it may have been under the subordination of either Split or Durazzo, both then Byzantine. The early Ras church can be dated to the 9th–10th century, with the rotunda plan characteristic of first court chapels. The bishopric was established shortly after 871, during the rule of Mutimir, and was part of the general plan of establishing bishoprics in the Slav lands of the empire, confirmed by the Council of Constantinople in 879–880. The names of Serbian rulers through Mutimir (r. 851–891) are Slavic dithematic names, per the Old Slavic tradition. With Christianization in the 9th century, Christian names appear. The next generations of Serbian royalty had Christian names (Petar, Stefan, Pavle, Zaharije, etc.), evident of strong Byzantine missions in the 870s. Petar Gojniković (r. 892–917) was evidently a Christian prince, and Christianity presumably was spreading in his time; also since Serbia bordered Bulgaria, Christian influences and perhaps missionaries came from there, increasing during the twenty-year peace. The Bulgarian annexation of Serbia in 924 was important for the future direction of the Serbian church, and by then, at latest, Serbia must have received the Cyrillic alphabet and Slavic religious text, already familiar but perhaps not yet preferred to Greek.
Archbishopric of Ohrid (1018–1219)
After the Byzantine conquest of Bulgaria in 1018, by order of emperor Basil II an autonomous Bulgarian Archbishopric of Ohrid was established in 1019, by lowering the rank of the autocephalous Bulgarian Patriarchate due to its subjugation to Constantinople, placing it under the supreme ecclesiastical jurisdiction of the Patriarchate of Constantinople. Gradually, Greek replaced Bulgarian Slavic as the liturgical language. Serbia was ecclesiastically administered into several bishoprics: the bishopric of Ras, mentioned in the first charter of Basil II (r. 976–1025), became part of the Ohrid archbishopric and encompassed the central areas of Serbia, by the rivers Raška, Ibar and Lim, evident in the second charter of Basil II. In the chrysobulls of Basil II dated to 1020, the Ras bishopric is mentioned as serving the whole of Serbia, with the seat at the Church of the Holy Apostles Peter and Paul, Ras. Among the first bishops were Leontius (fl. 1123–1126), Cyril (fl. 1141–1143), Euthemius (fl. 1170) and Kalinik (fl. 1196). It later joined the autocephalous Archbishopric of Žiča in 1219, at the time of Saint Sava.
The 10th- or 11th-century Gospel Book Codex Marianus, written in Old Church Slavonic in the Glagolithic script, is one of the oldest known Slavic manuscripts and was partly written in the Serbian redaction of Old Church Slavonic. Other early manuscripts include 11th-century
Grškovićev odlomak Apostola and