Under Boris I, Bulgaria became officially Christian, and the Ecumenical Patriarch agreed to allow an autonomous Bulgarian Archbishop at Pliska. Missionaries from Constantinople, Cyril and Methodius, devised the Glagolitic alphabet, which was adopted in the Bulgarian Empire around 886. The alphabet and the Old Bulgarian language that evolved from Slavonic^[27] gave rise to a rich literary and cultural activity centered around the Preslav and Ohrid Literary Schools, established by order of Boris I in 886.

In the early 9th century, a new alphabet — Cyrillic — was developed at the Preslav Literary School, adapted from the Glagolitic alphabet invented by Saints Cyril and Methodius.^[28] An alternative theory is that the alphabet was devised at the Ohrid Literary School by Saint Climent of Ohrid, a Bulgarian scholar and disciple of Cyril and Methodius.