The Slavs were Christianized in waves from the 7th to 12th century, though the process of replacing old Slavic religious practices began as early as the 6th century. Generally speaking, the monarchs of the South Slavs adopted Christianity in the 9th century, the East Slavs in the 10th, and the West Slavs between the 9th and 12th century. Saints Cyril and Methodius are attributed as "Apostles to the Slavs", having introduced the Byzantine-Slavic rite and Glagolitic alphabet, the oldest known Slavic alphabet and basis for the Early Cyrillic alphabet.

The Slavs or Slavic people are a group of peoples who speak Slavic languages. Slavs are geographically distributed throughout the northern parts of Eurasia; they predominantly inhabit Central Europe, Eastern Europe, and Southeastern Europe, though there is a large Slavic minority scattered across the Baltic states, Northern Asia, and Central Asia, and a substantial Slavic diaspora in the Americas, Western Europe, and Northern Europe.