Final answer:
St. Cyril and St. Methodius were the two Byzantine missionaries who created the written language for the Slavs, contributing to the spread of Christianity and cultural unity in Eastern Europe.
Step-by-step explanation:
The two Byzantine missionaries who developed a written language for the Slavic people of Russia were St. Cyril and his brother St. Methodius. They invented the Glagolitic alphabet, which was later adapted into the Cyrillic alphabet, now used in many Slavic languages including Russian. Their work greatly contributed to the spread of Eastern Orthodox Christianity and the cultural and religious unity of the Slavic peoples, tying together territory that extended deep into what would one day become Russia.
St. Cyril's efforts to translate Greek liturgy into Slavonic and his use of this language to teach and convert the inhabitants of Moravia and Bulgaria were instrumental in the missionary efforts that connected Eastern Europe and Byzantium. This spread of Christianity, leveraged by the prestige and power of the Byzantine Empire, was a pivotal moment in the cultural and religious development of the region, especially as seen in the conversion of Prince Vladimir I of Kiev, which led to mass baptism and the adoption of Byzantine art and architecture in Kievan Rus'.