6 Much of their success was due to Cyril's knowledge of the Slavic language, which he had acquired earlier. The influence of the Old and New Testaments as translated by him into the vernacular was also considerable. The whole Slavic people owe much to the man who gave them the Christian faith and with it the advantages of civilization. Cyril and Methodius were also the inventors of the alphabet which afforded the Slavic tongue the means for a written language, and they are even looked upon as having formed the language. Journey to Rome
Source: Grande Munus (Vatican.va)