Original sentence: The Greek missionaries had great success in part because they used the people's native language rather than Latin as the Roman priests did, or Greek.This work was made possible by the work of Saints Cyril and Methodius, who translated the Bible and many of the prayer books into the Slavonic vernacular, which became the literary language Old Church Slavonic
Edited sentence: Slavic missionaries had great success in part because they used the people's native language rather than Latin as the Roman priests did, or Greek.This work was made possible by the work of Saints Cyril and Methodius, who translated the Bible and many of the prayer books into the Slavonic vernacular of the Slavs of the region of Thessaloniki, which became the literary language Old Church Slavonic