The name ‘Orthodox Christianity’ refers to the form of Christianity brought from Byzantium to the peoples of Eastern Europe. With the division of the Roman Empire, the two capitals, Rome and Constantinople, rivalled each other and grew apart. The schism between the Roman Catholic Church and the Byzantine-Orthodox Church occurred in 1054 CE over a theological dispute concerning the nature of the Holy Spirit.

The conversion of slaves began with the migrations to the Balkans in the 6th and 7th centuries CE. Instead of Greek, the missionaries used local languages to spread their message. Thus, two brothers, Cyril (826-69) and Methodius (ca. 815-85), travelled to the modern-day Czech Republic, then Moravia, and translated the Bible and liturgical books to Slavic. To disseminate the Bible, they created an alphabet, known as the Glagolitic alphabet (from glagol, which means ‘word’ in Old Bulgarian). It was later replaced by the Cyrillic alphabet, named after Cyril, used by languages such as Bulgarian, Serbian, Ukrainian and Russian today. Romanian kept the Latin alphabet, although it remained under the influence of Orthodox Christianity.

Other Eastern European countries, in which Roman Catholic missionaries preached, adopted the Latin liturgy and remained under the centralized control of the Catholic Church. This group included the Croatians, Slovenians, Czechs, Slovakians, Poles, Lithuanians and Hungarians.

In 980 CE, Orthodox Christianity was established in Russia. The main centre of Slavic Orthodoxy was Kiev, in Ukraine; however, it was replaced by Moscow when Kiev fell under Mongol rule in 1237 CE.