Constantine and Methodius were dedicated to the ideal of expression in a people's native language. Throughout their lives they would battle against those who saw value only in Greek or Latin. Before they even left on their mission, tradition says, Constantine constructed a script for Slavonic -- a script that is known today as glagolithic. Glagolithic is considered by some as the precursor of cyrillic which named after him.
Arriving in 863 in Moravia, Constantine began translating the liturgy into Slavonic. In the East, it was a normal procedure to translate liturgy into the vernacular. As we know, in the West the custom was to use Greek and later Latin, until Vatican II. The German hierarchy, which had power over Moravia, used this difference to combat the brothers' influence. The German priests didn't like losing their control and knew that language has a great deal to do with independence.
So when Constantine and Methodius went to Rome to have the Slav priesthood candidates ordained (neither was a bishop at the time), they had to face the criticism the Germans had leveled against them. But if the Germans had motives that differed from spiritual concerns, so did the pope. He was concerned about the Eastern church gaining too much influence in the Slavic provinces. Helping Constantine and Methodius would give the Roman Catholic church more power in the area. So after speaking the brothers, the pope approved the use of Slavonic in services and ordained their pupils.
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