Despite a number of military setbacks, the reign of Boris I was marked with significant events that shaped Bulgarian and European history. With the Christianization of Bulgaria in 864 paganism (i.e. Tengrism) was abolished. A skillful diplomat, Boris I successfully exploited the conflict between the Patriarchate of Constantinople and the Papacy to secure an autocephalous Bulgarian Church, thus dealing with the nobility's concerns about Byzantine interference in Bulgaria's internal affairs.
When in 885 the disciples of Saints Cyril and Methodius were banished from Great Moravia, Boris I gave them refuge and provided assistance which saved the Glagolithic and later promoted the development of the Cyrillic script in Preslav and the Slavic literature. After he abdicated in 889, his eldest son and successor tried to restore the old pagan religion but was deposed by Boris I. During the Council of Preslav which followed that event, the Byzantine clergy was replaced with Bulgarians, and the Greek language was replaced with what is now known as Old Church Slavonic.