The establishment of Christianity as state religion dates to the time of Prince Mutimir and Byzantine Emperor Basil I (r. 867–886), who, after managing to put the Serbs under his nominal rule, sent priests along with admiral Niketas Ooryphas, before the operations against the Saracens in 869 when Dalmatian fleets were sent to defend the town of Ragusa.

The Christianization was due partly to Byzantine and subsequent Bulgarian influence.
It is important to note that at least during the rule of Kotsel of Pannonia (861–874), communications between Serbia and Great Moravia must have been possible.
The pope was presumably aware of this fact when planning Methodios' diocese.
The Dalmatian coast 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 is accounted Christian as of about 870.

The first Serbian bishopric was founded at the political center at Ras, near modern Novi Pazar on the Ibar river.
The initial affiliation is uncertain--it may have been under the subordination of either Split or Durazzo, both then Byzantine.
The early church of Saint Apostles Peter and Paul at Ras 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 Eparchy of Braničevo was founded in 878 (as a continuation of Viminacium and Horreum Margi).

The seal of Strojimir (d. between 880 and 896), the brother of Mutimir, was bought by the Serbian state in an auction in Germany.
The seal has a Patriarchal cross in the center and Greek inscriptions that say: "God, help Strojimir (CTPOHMIP)".

Petar Gojniković (r. 892–917) was evidently a Christian prince.
Christianity presumably was spreading in his time.
Also, since Serbia bordered Bulgaria, Christian influence--and perhaps missionaries--came from there.
This would increase during the twenty-year peace.
The previous generation (Mutimir, Strojimir and Gojnik) had Slav names, but the following (Petar, Stefan, Pavle, Zaharija) had Christian names, an indication of strong Byzantine missions to Serbia, as well as to the Slavs of the Adriatic coast, in the 870s.

The Bulgarian annexation of Serbia in 924 was important for the future direction of the Serbian church.
By then, at the latest, Serbia must have received the Cyrillic alphabet and Slavic religious text, already familiar but perhaps not yet preferred to Greek.

Notable early church buildings include the monastery of Archangel Michael in Prevlaka (Ilovica), built in the beginning of the 9th century, on the location of older churches of three-nave structure with three apses to the East, dating from the 3rd and 6th centuries, Bogorodica Hvostanska (6th century) and Church of Saints Peter and Paul.
The establishment of Christianity as state religion dates to the time of Prince Mutimir and Byzantine Emperor Basil I (r. 867–886), who, after managing to put the Serbs under his nominal rule, sent priests along with admiral Niketas Ooryphas, before the operations against the Saracens in 869 when Dalmatian fleets were sent to defend the town of Ragusa.
The Christianization was due partly to Byzantine and subsequent Bulgarian influence. It is important to note that at least during the rule of Kotsel of Pannonia (861–874), communications between Serbia and Great Moravia must have been possible. The pope was presumably aware of this fact when planning Methodios' diocese. The Dalmatian coast 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 is accounted Christian as of about 870.
The first Serbian bishopric was founded at the political center at Ras, near modern Novi Pazar on the Ibar river. The initial affiliation is uncertain--it may have been under the subordination of either Split or Durazzo, both then Byzantine. The early church of Saint Apostles Peter and Paul at Ras 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 Eparchy of Braničevo was founded in 878 (as a continuation of Viminacium and Horreum Margi).
The seal of Strojimir (d. between 880 and 896), the brother of Mutimir, was bought by the Serbian state in an auction in Germany. The seal has a Patriarchal cross in the center and Greek inscriptions that say: "God, help Strojimir (CTPOHMIP)".
Petar Gojniković (r. 892–917) was evidently a Christian prince. Christianity presumably was spreading in his time. Also, since Serbia bordered Bulgaria, Christian influence--and perhaps missionaries--came from there. This would increase during the twenty-year peace. The previous generation (Mutimir, Strojimir and Gojnik) had Slav names, but the following (Petar, Stefan, Pavle, Zaharija) had Christian names, an indication of strong Byzantine missions to Serbia, as well as to the Slavs of the Adriatic coast, in the 870s.
The Bulgarian annexation of Serbia in 924 was important for the future direction of the Serbian church. By then, at the latest, Serbia must have received the Cyrillic alphabet and Slavic religious text, already familiar but perhaps not yet preferred to Greek.
Notable early church buildings include the monastery of Archangel Michael in Prevlaka (Ilovica), built in the beginning of the 9th century, on the location of older churches of three-nave structure with three apses to the East, dating from the 3rd and 6th centuries, Bogorodica Hvostanska (6th century) and Church of Saints Peter and Paul.
Principality of Serbia (early medieval)