The apostles of Bulgaria are Saint Cyril and Saint Methodius, who conducted Christianization missions in the 9th century, including: in the lands inhabited by Slavs.

An excerpt from the article 34 facts about Bulgaria

The brothers simplified and modified the 38-letter alphabet reflecting the sounds found in the Old Church Slavonic language, called the Glagolitic alphabet. It is the oldest known Slavic writing. Glagolitic was a phonetic script that was finally developed in 863. Later, it was gradually replaced by the Cyrillic alphabet (its creation is attributed to the disciples of Cyril and Methodius), which was an alphabetic script used to write the Eastern Slavic languages, most of the South Slavic languages, and others. By the 12th century, the Cyrillic alphabet spread from Bulgaria to Serbia and Kievan Rus.