After the Christianization of Bulgaria in 864, Saint Clement of Ohrid and Saint Naum of Preslav were of great importance to the Orthodox faith and the Old Church Slavonic liturgy in the First Bulgarian Empire. The success of the conversion of the Bulgarians facilitated the conversion of East Slavic peoples, most notably the Rus', predecessors of Belarusians, Russians, and Ukrainians. A major event was the development of the Cyrillic script in Bulgaria at the Preslav Literary School in the 9th century. The Cyrillic script and the liturgy in Old Church Slavonic, also called Old Bulgarian, were declared official in Bulgaria in 893.