9 The Slavonic Life of Methodius reports in the following words the request made by the Prince Rastislav to the Emperor Michael III through his envoys: "Many Christian teachers have reached us from Italy, from Greece and from Germany, who instruct us in different ways. But we Slavs ... have no one to direct us towards the truth and instruct us in an understandable way". It was then that Constantine and Methodius were invited to go there. Their profoundly Christian response to the invitation in this circumstance and on all similar occasions is admirably expressed by the words of Constantine to the Emperor: "However tired and physically worn out I am, I will go with joy to that land"; "with joy I depart for the sake of the Christian faith". The truth and the power of their missionary mandate came from the depths of the mystery of the Redemption, and their evangelizing work among the Slav peoples was to constitute an important link in the mission entrusted by the Savior to the Church until the end of time. It was a fulfillment - in time and in concrete circumstances - of the words of Christ, who in the power of his Cross and Resurrection told the Apostles: "Preach the Gospel to the whole creation"; "Go therefore and make disciples of all nations". In so doing, the preachers and teachers of the Slav peoples let themselves be guided by the apostolic ideal of Saint Paul: "For in Christ Jesus you are all children of God, through faith. For as many of you as were baptized into Christ have put on Christ. There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus". Together with a great respect for persons and a disinterested concern for their true good, the two holy Brothers had the resources of energy, prudence, zeal and charity needed for bringing the light to the future believers, and at the same time for showing them what is good and offering concrete help for attaining it. For this purpose they desired to become similar in every aspect to those to whom they were bringing the Gospel; they wished to become part of those peoples and to share their lot in everything.
Source: Slavorum Apostoli (Vatican.va)