Saints Cyril and Methodius in the context of "Duchy of Bohemia"

Old Church Slavonic (OCS) or Old Slavonic (/sləˈvɒnɪk, slæˈvɒn-/ slə-VON-ik, slav-ON-) is the first Slavic literary language and the oldest extant written Slavonic language attested in literary sources. It belongs to the South Slavic subgroup of the Balto-Slavic branch of the Indo-European language family and remains the liturgical language of many Christian Orthodox churches.

Historians credit the 9th-century Byzantine missionaries Saints Cyril and Methodius with standardizing the language and undertaking the task of translating the Gospels and necessary liturgical books into it as part of the Christianization of the Slavs. It is thought to have been based primarily on the dialect of the 9th-century Byzantine Slavs living in the Province of Thessalonica (in present-day Greece).

The Slavs were Christianized in waves from the 7th to 12th century, though the process of replacing old Slavic religious practices began as early as the 6th century. Generally speaking, the monarchs of the South Slavs adopted Christianity in the 9th century, the East Slavs in the 10th, and the West Slavs between the 9th and 12th century. Saints Cyril and Methodius (fl. 860–885) are attributed as "Apostles to the Slavs", having introduced the Byzantine-Slavic rite (Old Slavonic liturgy) and Glagolitic alphabet, the oldest known Slavic alphabet and basis for the Early Cyrillic alphabet.

The simultaneous missionary efforts to convert the Slavs by what would later become known as the Catholic Church of Rome and the Eastern Orthodox Church of Constantinople led to a 'second point of contention between Rome and Constantinople', especially in Bulgaria (9th–10th century). This was one of many events that preceded the East–West Schism of 1054 and led to the eventual split between the Greek East and Latin West. The Slavs thus became divided between Eastern Orthodoxy and Roman Catholicism. Closely connected to the competing missionary efforts of the Roman Church and the Byzantine Church was the spread of the Latin and Cyrillic scripts in Eastern Europe. The majority of Orthodox Slavs adopted Cyrillic, while most Catholic Slavs adopted the Latin, but there were many exceptions to this general rule. In areas where both Churches were proselytising to pagan Europeans, such as the Grand Duchy of Lithuania, the Croatian Duchy and the Principality of Serbia, mixtures of languages, scripts and alphabets emerged, and the lines between Latin Catholic (Latinitas) and Cyrillic Orthodox literacy (Slavia Orthodoxa) were blurred.

Great Moravia (Latin: Regnum Marahensium; Greek: Μεγάλη Μοραβία, Meghálī Moravía; Czech: Velká Morava [ˈvɛlkaː ˈmorava]; Slovak: Veľká Morava [ˈvɛʎkaː ˈmɔrava]; Polish: Wielkie Morawy, German: Großmähren), or simply Moravia, was the first major state that was predominantly West Slavic to emerge in the area of Central Europe, possibly including territories which are today part of the Czech Republic, Slovakia, Hungary, Austria, Germany, Poland, Romania, Croatia, Serbia, Ukraine and Slovenia. The formations preceding it in these territories were Samo's tribal union (631–658) and the Pannonian Avar state (567–822).

Centered on the Morava River – which gave the realm its name – the core encompassed today’s Moravia in the eastern Czech Republic and adjacent western Slovakia. The kingdom saw the rise of the first-ever Slavic literary culture in the Old Church Slavonic language as well as the expansion of Christianity, first via missionaries from East Francia, and later after the arrival of Saints Cyril and Methodius in 863 and the creation of the Glagolitic alphabet, the first alphabet dedicated to a Slavic language. Glagolitic was subsequently replaced by the Cyrillic alphabet created in the First Bulgarian Empire.

Christianity in the Middle Ages covers the history of Christianity from the fall of the Western Roman Empire (c. 476). The end of the period is variously defined - depending on the context, events such as the conquest of Constantinople by the Ottoman Empire in 1453, Christopher Columbus's first voyage to the Americas in 1492, or the Protestant Reformation in 1517 are sometimes used.

In Christianity's ancient Pentarchy, five patriarchies held special eminence: the sees of Rome, Constantinople, Jerusalem, Antioch, and Alexandria. The prestige of most of these sees depended in part on their apostolic founders, or in the case of Byzantium/Constantinople, that it was the new seat of the continuing Eastern Roman, or Byzantine Empire. These bishops considered themselves the successors of those apostles. In addition, all five cities were early centres of Christianity, losing their importance after the Levant was conquered by the Sunni Caliphate.

Slavic paganism, Slavic mythology, or Slavic religion refer to the religious beliefs, myths, and ritual practices of the Slavs before Christianisation, which occurred at various stages between the 8th and the 13th century.

The South Slavs, who likely settled in the Balkans during the 6th–7th centuries AD, bordering with the Byzantine Empire to the south, came under the sphere of influence of Eastern Christianity relatively early, beginning with the creation of writing systems for Slavic languages (first Glagolitic, and then Cyrillic script) in 855 by the brothers Saints Cyril and Methodius and the adoption of Christianity in Bulgaria in 864 and 863 in Great Moravia. The East Slavs followed with the official adoption in 988 by Vladimir the Great of Kievan Rus'.

The Glagolitic script (/ˌɡlæɡəˈlɪtɪk/ GLAG-ə-LIT-ik, ⰳⰾⰰⰳⱁⰾⰻⱌⰰ, glagolitsa) is the oldest known Slavic alphabet. It is generally agreed that it was created in the 9th century for the purpose of translating liturgical texts into Old Church Slavonic by Saint Cyril, a monk from Thessalonica. He and his brother Saint Methodius were sent by the Byzantine Emperor Michael III in 863 to Great Moravia after an invitation from Rastislav of Moravia to spread Christianity there. After the deaths of Cyril and Methodius, their disciples were expelled from Moravia, and they moved to the First Bulgarian Empire instead. The Early Cyrillic alphabet, which was developed gradually in the Preslav Literary School by scribes who incorporated some Glagolitic letters when writing in the Greek alphabet, gradually replaced Glagolitic in that region. Glagolitic remained in use alongside the Latin script in the Kingdom of Croatia and alongside Cyrillic until the 14th century in the Second Bulgarian Empire and the Serbian Empire; in later periods, it was used mainly for cryptographic purposes.

Glagolitic also spread to the Kievan Rus' and the Kingdom of Bohemia. Although its use declined there in the 12th century, some manuscripts in the territory of the former retained Glagolitic inclusions for centuries. It had also spread to Duklja and Zachlumia in the Western Balkans, from where it reached the March of Verona. There, the Investiture Controversy afforded it refuge from the opposition of Latinizing prelates and allowed it to entrench itself in Istria, from which place it spread to nearby lands. It survived there and as far south as Dalmatia without interruption into the 20th century for Church Slavonic in addition to its use as a secular script in parts of its range, which at times extended into Bosnia, Slavonia, and Carniola, in addition to 14th–15th century exclaves in Prague and Kraków, and a 16th-century exclave in Putna.