Kievan Rus ' is important for its introduction of a Slavic variant of the Eastern Orthodox religion, dramatically deepening a synthesis of Byzantine and Slavic cultures that defined Russian culture for the next thousand years.

This is also true in the Slavic languages following the Eastern Orthodox tradition ( e. g. "" ( Iisús Navín ) in Russian ).

In the Slavic tradition of the Eastern Orthodox Church, these two are commemorated on 19 November ( corresponding to 2 December on the Gregorian calendar ).

They brought with them the Old Church Slavonic liturgy of the Eastern Orthodox Christian religion, written Slavic language, the version of which known as Chancery Slavonic was to serve the Lithuanian court's document-producing needs for a few centuries, and developed laws, turning Vilnius into a major center of their civilization.

In his dealings with Constantinople in the matter of Photius, as also in his relations with the young Slavic Orthodox church, he pursued the policy of Pope Nicholas I.

In 988 it adopted Orthodox Christianity from the Byzantine Empire, beginning the synthesis of Byzantine and Slavic cultures that defined Russian culture for the next millennium.

The reigns of Vladimir the Great ( 980 – 1015 ) and his son Yaroslav the Wise ( 1019 – 1054 ) constitute the Golden Age of Kiev, which saw the acceptance of Orthodox Christianity from Byzantium and the creation of the first East Slavic written legal code, the Russkaya Pravda.

Cyrillic eventually spread throughout most of the Slavic world to become the standard alphabet in the Orthodox Slavic countries.

The capital Preslav was said to rival Constantinople, the new independent Bulgarian Orthodox Church became the first new patriarchate besides the Pentarchy and Bulgarian translations of Christian texts spread all over the Slavic world of the time.

The common cultural bond of Eastern Orthodox Christianity and written Church Slavonic ( a literary and liturgical Slavic language developed by 8th century missionaries Saints Cyril and Methodius ) fostered the emergence of a new geopolitical entity, Kievan Rus ' — a loose-knit network of principalities, established along preexisting trade routes, with major centers in Novgorod ( currently Russia ), Polatsk ( in Belarus ) and Kiev ( currently in Ukraine ) — which claimed a sometimes precarious preeminence among them.

It played an important role in the history of the Slavic languages and served as a basis and model for later Church Slavonic traditions, and some Eastern Orthodox and Eastern Catholic churches use Church Slavonic as a liturgical language to this day.

Apart from the Slavic countries, Slavonic has been used as a liturgical language by the Romanian Orthodox Church, as well as a literary and official language of the prince courts of Wallachia and Moldavia ( see Old Church Slavonic in Romania ), before gradually being replaced by Romanian starting with the 18th century.

The Oriental Orthodox generally follow the pattern of the Slavic tradition with respect to the archbishop / metropolitan distinction.

The Eastern Orthodox Church calls dioceses metropoleis in the Greek tradition or eparchies in the Slavic tradition.

Rus ' had accepted the Orthodox Christianity from the East Roman Empire in 988, and this largely defined the Russian culture of next millennium as the synthesis of Slavic and Byzantine cultures.

The Bosniaks, whose ethnonym initially referred to Slavic Christians ( Orthodox, Catholic and Bogomils ) which co-existed in Bosnia and Herzegovina in the Middle Ages, are today majority Muslim, since the Ottoman occupation.

Although the Serbs and Bulgarians share Slavic kinship, Orthodox Christianity and cultural traits, the two peoples have been relatively hostile against each other in history and were early on understood as distinct ethnic groups.

The Serbs are predominantly Orthodox Christian, and before Christianity, they adhered to Slavic paganism.

In Eastern Orthodox and Greek-Catholic Churches of Slavic tradition, the Gospel account of Zacchaeus is read on the last Sunday preceding the liturgical preparation for Great Lent, for which reason that Sunday is known as " Zacchaeus Sunday.

Although Europe had already experience the East-West Schism by this time, such a split was far less concrete than it is today, and Catholic Slavs in Bosnia and the Dalmatian coast practiced Christianity in a similar way to Orthodox Slavs – priests married, wore beards and gave liturgy in Slavic rather than Latin.

Bulgaria functioned as the hub of Slavic Europe during much of the Middle Ages, exerting considerable literary and cultural influence over the Eastern Orthodox Slavic world by means of the Preslav and Ohrid Literary Schools.