About 860 CE, Eastern Orthodox Christian missionaries Constantine (later St. Cyril) and his brother Methodius invented the Glagolitic alphabet for Slavic languages. Originating from Thessalonica on the southern edge of the Slavic-speaking world, these missionaries created the round Glagolitic script (Glagolitsa), the oldest known Slavic alphabet. In 862, Prince Rostislav of Great Moravia requested Constantinople send missionaries for Slavic Christianization. Using Greek script as a model, Cyril and Methodius developed this script specifically for Old Church Slavonic to translate the Bible and liturgical documents, enabling Christianization of Slavic peoples who were previously pagan.

Cyril and Methodius used Glagolitic to translate the Bible and liturgical documents from Greek into Old Church Slavonic, creating the first Slavic literary language. Their disciples spread Glagolitic to Bosnia, Croatia, Serbia, and Lesser Poland, establishing Slavic Christian literature. This missionary linguistics work directly influenced later Slavic poetry, religious writing, and the development of Cyrillic script by their disciples in Bulgaria and Macedonia during the 900s.