[9] For the next 22 years, he worked with Cyril and Methodius and other missionaries in translating the Bible into Old Church Slavonic and promoted it in Great Moravia and Principality of Lower Pannonia.

In 867 or 868 he became a priest in Rome, ordained along with two other disciples of Cyril and Methodius, Gorazd and Clement of Ohrid, by bishops Formosus and Gauderic.

After a brief period of imprisonment due to the ongoing conflict with the German clerics, Naum, together with some of the missionaries (including Clement of Ohrid and Angelar) headed to Bulgaria.

Fearing growing Byzantine influence Boris viewed the adoption of the Old Church Slavonic as a way to preserve the political independence of Bulgaria.

The development of Old Church Slavonic literacy had the effect of preventing the assimilation into the neighboring cultures and promoted the formation of a distinct Bulgarian identity.

In 893, shortly after his rise to power, the new Bulgarian ruler Simeon the Great, summoned an ecclesiastical council in the new capital Preslav, where Clement was ordained bishop of Drembica and Velika.

In these years the Cyrillic script was created in the Preslav literary school,[12] and was adopted in Bulgaria, possibly following Naum's initiative.

Saint Naum's original feast day was December 23, but in 1727 on the authority of the Archbishop of Ohrid, it was changed to June 20.