Eastern Orthodox Church

=== Conversion of South and East Slavs ===

{{Further|Christianisation of Bulgaria|Christianisation of Kievan Rus'}}

In the ninth and tenth centuries, Christianity made great inroads into pagan Europe, including [[First Bulgarian Empire|Bulgaria]] (864) and later [[Kievan Rus']] (988). This work was made possible by  [[Cyril and Methodius]] of [[Thessaloniki]], two brothers chosen by [[Michael III|Byzantine emperor Michael III]] to fulfil the request of [[Rastislav of Moravia]] for teachers who could minister to the Moravians in their own language. Cyril and Methodius began translating the [[divine liturgy]], other liturgical texts, and the [[Gospels]] along with some other [[Bible|scriptural texts]] into local languages; with time, as these translations were copied by speakers of other dialects, the hybrid literary language [[Church Slavonic]] was created. Originally sent to convert the Slavs of [[Great Moravia]], Cyril and Methodius were forced to compete with [[Frankish mythology|Frankish]] missionaries from the Roman diocese; their [[Saint Sava (disciple of Saints Cyril and Methodius)|disciples]] were driven out of Great Moravia in AD 886 and emigrated to [[First Bulgarian Empire|Bulgaria]].<ref>A. Avenarius. Christianity in 9th-century Rus. // Beitruge zur byzantinischen Geschichte im 9.-11. Jahrhundert. Prague: V. Vavrinek, 1978. pp. 301–315.</ref>

[[File:Radzivill Olga in Konstantinopol.jpg|thumb|upright=1.25|''The baptism of [[Olga of Kiev|Princess Olga]] in [[Constantinople]]'', a miniature from the ''[[Radziwiłł Chronicle]]'']]

After the [[Christianisation of Bulgaria]] in 864, [[Saint Angelar|the disciples]] of [[Cyril and Methodius]] in [[First Bulgarian Empire|Bulgaria]], the most important being [[Clement of Ohrid]] and [[Saint Naum|Naum of Preslav]], were of great importance to the Orthodox faith in the [[First Bulgarian Empire]]. In a short time they managed to prepare and instruct the future [[First Bulgarian Empire|Bulgarian]] clergy into the biblical texts and in AD 870 the [[Fourth Council of Constantinople (Roman Catholic)|Fourth Council of Constantinople]] granted the Bulgarians the oldest organised [[autocephalous]] Slavic Orthodox Church, which shortly thereafter became Patriarchate. The success of the [[Christianisation of Bulgaria|conversion of the Bulgarians]] facilitated the conversion of the [[East Slavs]].<ref>{{cite web|url=http://www.stclementofohrid.com/religion/st_clement_of_ohrid.asp|title=St. Clement of Ohrid Cathedral – About Saint Clement of Ohrid|author=Aco Lukaroski|access-date=5 March 2015|archive-date=16 May 2015|archive-url=https://web.archive.org/web/20150516024053/http://www.stclementofohrid.com/religion/st_clement_of_ohrid.asp|url-status=dead}}</ref>{{failed verification|date=September 2023}} A major event in this effort was the development of the [[Cyrillic script]] in [[Bulgaria]], at the [[Preslav Literary School]] in the ninth century; this script, along with the liturgical [[Old Church Slavonic]], also called [[Old Bulgarian]], was declared official in [[Bulgaria]] in 893.<ref>{{cite book | first=Francis | last=Dvornik |title=The Slavs: Their Early History and Civilization | quote = The Psalter and the Book of Prophets were adapted or "modernised" with special regard to their use in Bulgarian churches, and it was in this school that glagolitic writing was replaced by the so-called Cyrillic writing, which was more akin to the Greek uncial, simplified matters considerably and is still used by the Orthodox Slavs. | year = 1956 | place = Boston | publisher = American Academy of Arts and Sciences | page = 179}}</ref><ref>{{cite book|url=https://archive.org/details/southeasterneuro0000curt |url-access= registration |quote= Cyrillic preslav. |title= Southeastern Europe in the Middle Ages, 500–1250 |series= Cambridge Medieval Textbooks |author= Florin Curta |publisher= Cambridge University Press |year= 2006 |isbn=978-0-521-81539-0|pages= [https://archive.org/details/southeasterneuro0000curt/page/221 221]–222}}</ref><ref>{{cite book|chapter-url=https://books.google.com/books?id=J-H9BTVHKRMC&q=The+Orthodox+Church+in+the+Byzantine+Empire+Cyrillic+preslav+eastern&pg=PR3-IA34|chapter= The Orthodox Church in the Byzantine Empire|title= Oxford History of the Christian Church|author= J. M. Hussey, Andrew Louth|publisher= Oxford University Press|year= 2010|isbn=978-0-19-161488-0|pages= 100}}</ref>

The work of Cyril and Methodius and their disciples had a major impact on the [[Serbs]] as well.<ref name=Cyril>''Columbia Encyclopedia'', Sixth Edition. 2001–05, s.v. "Cyril and Methodius, Saints".</ref><ref>''Encyclopædia Britannica'', Warren E. Preece – 1972, p. 846, s.v., "Cyril and Methodius, Saints" and "Orthodoxy, Missions ancient and modern".</ref><ref>''Encyclopedia of World Cultures'', David H. Levinson, 1991, p. 239, s.v., "Social Science".</ref><ref>Eric M. Meyers, ''The Oxford Encyclopedia of Archaeology in the Near East'', p. 151, 1997.</ref><ref>Lunt, ''Slavic Review'', June 1964, p. 216.</ref><ref>Roman Jakobson, "Crucial problems of Cyrillo-Methodian Studies".</ref><ref>Leonid Ivan Strakhovsky, ''A Handbook of Slavic Studies'', p. 98.</ref><ref>V. Bogdanovich, ''History of the ancient Serbian literature'', Belgrade, 1980, p. 119.</ref> They accepted Christianity collectively along familial and tribal lines, a gradual process that occurred between the seventh and ninth centuries. In commemoration of their baptisms, each Serbian family or tribe began to celebrate an exclusively Serbian custom called [[Slava (patron saint)]] in a special way to honour the saint on whose day they received the sacrament of [[baptism]]. It is the most solemn day of the year for all Serbs of the Orthodox faith and has played a role of vital importance in the history of the Serbian people. Slava remains a celebration of the conversion of the Serbian people, which the church blessed and proclaimed a church institution.<ref>{{cite book| author = Stephan Thernstrom| title = Harvard Encyclopedia of American Ethnic Groups| year = 1980| publisher = Belknap Press| isbn = 978-0-674-37512-3| page = [https://archive.org/details/harvardencyclope00ther/page/925 925]| url = https://archive.org/details/harvardencyclope00ther/page/925}}</ref>

The missionaries to the East and [[South Slavs]] had great success in part because they used the people's native language rather than [[Greek language|Greek]], the predominant language of the Byzantine Empire, or [[Latin]], as the Roman priests did.<ref name=Cyril/> Perhaps the greatest legacy of their efforts is the Russian Orthodox Church, which is the largest of the Orthodox churches.<ref>{{cite web|url=http://www.ucrdc.org/HA-RUSSIAN_ORTHODOX_CHURCH.html|title=Harvest of Despair|publisher=Ukrainian Canadian Research and Documentation Centre (UCRDC)|access-date=21 January 2019}}</ref>