Christianization of the Slavs
1. Byzantine church did great efforts to bring Christianity to them, Rome did the Same
2. The monarch of the South Slavs (Balkans) adopted Christianity in the 9th century
3. The East Slavs (Bulgars and Rus') did so in the 10th century
4. Saints Cyril and Methodius, who were both belonging the Byzantine Church and lived in the second half of the 9th century, were instrumental in that process (= Cyrillic alphabet)