It is thought to have been based primarily on the dialect of the 9th-century Byzantine Slavs living in the Province of Thessalonica (in present-day Greece). Historians credit the 9th-century Byzantine missionaries Saints Cyril and Methodius with standardizing the language and undertaking the task of translating the Gospels and necessary liturgical books into it as part of the Christianization of the Slavs. Old Church Slavonic or Old Slavonic ( / s l ə ˈ v ɒ n ɪ k, s l æ ˈ-/) is the first Slavic literary language.

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