After their arrival to Balkan Peninsula and acceptance of Christianity, Serbian people found itself on territories being, in terms of church jurisdiction, disputed between Constantinople and Rome. Already during reign of knyaz Mutimir (851 – 891), Serbs were becoming more inclined towards Constantinople and Byzantium. Letter from Pope John VIII sent to knyaz Mutimir in 873 confirms this. In this letter, Pope requests from Serbian knyaz to return his state, in ecclesiastical terms, under jurisdiction of Pannonia diocese with center in Sirmium (contemporary Sremska Mitrovica). 
However, struggle between Constantinople and Rome for ecclesial domination in the Balkans will last until the end of medieval period. In the first Split convocation from 925, ruler of Zachlumian Serbs, Mihajlo Višević, participated and it was during this convocation that the Roman Church strengthened its strongholds on the Dalmatian coast. Preserved fragment of the conclusion from this convocation reads: “And these episcopes, after previously visiting Dalmatian cities and meeting with Croatian and Serbian noblemen, arrived to the city of Split where, after meeting with judges (zhupans) and episcopes, held the famous assembly.”
Roman Church will establish two of its bishoprics, one in Zachlumia and other in Travunia, which will be referred to in Latin sources as „Zachulmie regnum“ i „regnum Tribunie“. These two regions very early became independent from the initial Serbia, thus special bishoprics where created in their territories.
Territory of the initial Bosnia was within early medieval Serbia. It was one of the of the Serbian administrative-territorial units – zhupa. Roman church had special bishopric for Serbia which, for a time period, was known in Latin sources as „regnum Seruilie“, which indicates that territory of initial Bosnia fell under its jurisdiction. Paragraph from two charters by Pope Callixtus II from September of 1120 reads:“„videlicet: Zachulmie regnum, et regnum Seruilie, Tribunieque regnum, ciuitas quoque Catharinensis seu Rose, Buduanensis Auarorum, Liciniatensis, Scodrinensis, Driuastensis et Polatensis, cum abbaciis, ecclesiis et parochiis earum“.
Besides influence of the Catholic Church in Serbian ethnic area of the early medieval period, Byzantine influence could be perceived through work of the Ohrid archiepiscopate which represented the Eastern Constantinople church through its own church organization. Episcopates in Prizren, Ljipljan, Ras and others worked under its jurisdiction. This text will not deal with Eastern Church, so it will not go into details regarding presence of the Eastern Church in early medieval period amongst Serbs, it will only ascertain there was such presence, especially east of Drina, but in the areas west from that river as well.
Another Pope, Hadrian IV, presenting the archbishopric mantle to Tribun, the Archbishop of Dubrovnik, lists bishoprics subordinated to himself in following order: „regnum Seruilie scilicet, regnum Zachulmie ac regnum Tribunie, ciuitatem quoque Catharinensis seu Rose, Guduanensem Auarorum, Liciniatensem, Scodrinensem, Driuastinsem et Polatensem, cum abbatiis, ecclesiis et parochiis suis“.  We can notice here as well that Serbia is encompassed by only one bishopric.
When Stefan Nemanja gained power in Rascia, in both ecclesiastical and religious terms, he oriented his state towards Byzantium, laying foundations for final adoption of Orthodoxy within his country. Letter from Gregory, archbishop of Dioclea and Bar, confirms this. In this letter, sent by Gregory to Gualterio, canonic of Split and papal envoy, Gregory describes persecution faced by his church from Stefan Nemanja.  Also, in 1181, Pope Alexander III warns brother of Stefan Nemanja, Zachlumian knyaz Miroslav, not to endanger the rights of the Catholic Church in Dalmatia. Family of Nemanjići and overwhelming majority of Serbs east of Drina accepted Orthodoxy. In time when struggle between Eastern and Western Church, culminating in fall of Constantinople to the Crusaders in 1204, it was necessary to put in order ecclesiastical issues within the state and unify people and the country under one church organization. Catholic Church kept its influence mostly along the Serbian coast.
Unwilling to give up its pretensions on Serbia, Catholic Church strived to keep the church traditions of the earlier church organization of “regnum Seruilie” in a newer form which is, in Latin sources from the last quarter of the XII and good part of the XIII century, known as “regnum Seruilie quod est Bosna” which would translate as “that part of Serbia which is called Bosnia”. From this bishopric will later develop church organization known in domestic sources as “Bosnian church”.
Papal envoy Theobald in 1180 sends a letter to ban of Bosnia, Kulin, requesting from ban two servants and skin of marten, calling him Culin bano Bosine.  Several years later, papal charters start to mention Bosnia as part of Serbia under jurisdiction of the Archbishop of Dubrovnik. We can see that ban Kulin has contacts with Catholic Church which had its own ecclesiastical organization within Bosnia, while politically, Bosnia at that time is a vassal state to Catholic Hungary. Zhupan Vukan Nemanjić, in his charter to Pope from 1199 in which he submits to the Catholic church authority, complains about ban Kulin due to his divergence from dogmas of the Catholic Church and names Bosnia as one of the lands of the Hungarian king terra regis Ungarie, videlicet Bossina.
All this influenced that banate of Bosnia, that is, its bishopric called “regnum Seruilie quod est Bosna” receives all traditions of the early medieval Serbian Catholic bishopric. When Pope Urban III in Verona on 28th of March 1187 confirms the rights of the Dubrovnik church, among other issues, he emphasizes that “regnum Zachulmie, regnum Seruilie quod est Bosna ae regnum Tribunie”  fall under its jurisdiction. Next year, his successor Pope Clement III, in his charter guaranteeing old privileges of the Dubrovnik Archbishopric also mentions Bosnia as part of Serbia “regnum Seruilie quod est Bosna”.
During XIII century there are others papal charters mentioning Bosnia as part of Serbia in the Catholic sphere of influence. Pope Gregory IX on 24th July of 1227 confirms all privileges Dubrovnik Archbishop received from previous popes and among the names of bishoprics under jurisdiction of Dubrovnik, he mentions “regnum Seruilie quod est Bosna.”  Around 1245, Varadin canonic Ruđer writes how Tatars destroyed “destruxit Boznam, regnum Rasciae”. Here as well we see that Bosnia is mentioned as part of kingdom of Rascia, that is, Serbia. This is the time of ban Ninoslav reign when he himself also names his subjects in Bosnia as Serbs. 
Several years after death of ban Ninoslav, papal diplomacy uses the same term. Archbishop of Dubrovnik, Ivan, while in Perugia, on the 24th of February 1252, writes his report regarding discussion on church rights against Bar Archbishopric and again mentions „regnum Seruilie quod est Bosgna“. Archbishop Ivan writes: “Et regnum Zachulmie extedintur vsque ad prouinciam Spalatensem, regnum Seruilie extedintur vsque ad prouinciam Colloncesem, regnumTribunie extenditur vsque ad prouinciam Dirachinam“. Translation reads: “And while Zachlumian kingdom extended as far as Split province, kingdom of Serbia extended as far as Kolac province, while kingdom of Travunia extended as far as Dyrrachium province”.
Surely, kingdom mentioned here are not real but actually represent ecclesiastical regions given that this is time period when only Serbia is kingdom encompassing both Zachlumia and Travunia, while Bosnia is ruled by domestic bans and is under supreme rule of Hungary.
To make the struggle against Bosnian heretics, majority of population of Bosnia at that time from perspective of Catholic Church, more efficient, on the 26th of August 1247, Bosnian bishopric was removed from jurisdiction of Archbishop of Dubrovnik and placed under jurisdiction of Hungarian-Kolac Archbishopric, due to the fact Pope called upon Hungarian kings and bishops to lead a war against Bosnian heretics. 
When we know this, it is clear why letter of Archbishop Ivan mentions that “kingdom” of Serbia extended as far as Kolac province. Under kingdom of Serbia he implies territory of the initial Serbian state which remained in Catholic sphere of influence, due to Hungarian supreme authority, direct influence of Catholic Church and struggle against Bosnian schismatics.
All sources listed above indicated that after final acceptance of Orthodoxy and Byzantine influence in Rascia during the reigns of first Nemanjići rulers, traditions of Catholic Serbian bishopric was transferred to Bosnian bishopric as Bosnia was part of Serbia and kept its traditions developed under influence of the Catholic Church. Papal Latin charters and charters of other dignitaries of the Catholic Church, referring to Bosnian bishopric, are important source for history of medieval Bosnia. From Bosnian bishopric, which was identified as “regnum Seruilie quod est Bosna” in XII and XIII century, different state church organization will develop within medieval Bosnian state, known as “Bosnian church”. This indicates that during medieval period, Serbian people was separated in three different church structures. Until the first half of the XIV century it was divided between Eastern Orthodox and Western Catholic churches, but with creation of “Bosnian church” which was, as such, first time mentioned in XIV century  it got its third ecclesial organization.
Later on, Archbishopric of Bar will become responsible for Catholics in Serbia while Catholic province of “Bosna Srebrena” will develop on territory of Bosnia and will eventually extend beyond borders of Bosnia and Herzegovina.
Author: Boris Radaković
Translate Ljubisa Malenica
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