5 The event which was to determine the whole of the rest of their lives was the request made by Prince Rastislav of Greater Moravia to the Emperor Michael III, to send to his peoples "a Bishop and teacher ... able to explain to them the true Christian faith in their own language". Those chosen were Saints Cyril and Methodius, who readily accepted, set out and, probably by the year 863, reached Greater Moravia-a State then including various Slav peoples of Central Europe, at the crossroads of the mutual influences between East and West. They undertook among these peoples that mission to which both of them devoted the rest of their lives, spent amidst journeys, privations, sufferings, hostility and persecution, which for Methodius included even a period of cruel imprisonment. All of this they bore with strong faith and indomitable hope in God. They had in fact prepared well for the task entrusted to them: they took with them the texts of the Sacred Scriptures needed for celebrating the Sacred Liturgy, which they had prepared and translated into the Old Slavonic language and written in a new alphabet, devised by Constantine the Philosopher and perfectly adapted to the sounds of that language. The missionary activity of the two Brothers was accompanied by notable success, but also by the understandable difficulties which the preceding initial Christianization, carried out by the neighboring Latin Churches, placed in the way of the new missionaries. About three years later, while travelling to Rome, they stopped in Pannonia where the Slav Prince Kocel, who had fled from the important civil and religious center of Nitra, gave them a hospitable reception. From here, after some months, they set out again for Rome together with their followers, for whom they desired to obtain Holy Orders. Their route passed through Venice, where the innovating elements of the mission they were carrying out were subjected to a public discussion. In Rome Pope Hadrian II, who had in the meantime succeeded Nicholas I, received them very cordially. He approved the Slavonic liturgical books, which he ordered to be solemnly placed on the altar in the Church of Saint Mary ad Praesepe, today known as Saint Mary Major, and recommended that their followers be ordained priests. This phase of their efforts concluded in a most favorable manner. Methodius however had to carry out the next stages by himself, because his younger brother, now gravely ill, scarcely had time to take religious vows and put on the monastic habit before he died shortly afterwards, on 14 February 869 in Rome.
Source: Slavorum Apostoli (Vatican.va)