12 But this became a source of trial to Methodius. Some members of the new Christian community became jealous of his accomplishments and virtue. They accused him, to Pope John VIII, Adrian's successor, of being unsound in faith, though he was innocent. They also accused him of violating the traditions of the Fathers who used only the Latin or Greek languages in discharging their religious duties. Wanting to preserve the integrity of the faith and to maintain the ancient traditions, the pope then summoned Methodius to Rome to justify himself. Methodius appeared before Pope John, several bishops, and the Roman clergy in 880, for he was ever ready to obey and confident in the testimony of his conscience. He obtained an easy victory by proving that he had followed Catholic teachings himself. He showed that he had always taught others the faith which he had sworn on the tomb of St. Peter, the prince of the Apostles, an oath given in the presence of Adrian and with his approval. If he had used the Slavic language in the celebration of the sacraments, he had done so for good reasons, since he had the special permission of Pope Adrian himself and the Holy Scriptures did not forbid it. Methodius freed himself so completely from every suspicion of guilt that the pope embraced him then and there and confirmed his archiepiscopal jurisdiction and his mission to the Slavs. Methodius returned to Moravia in the company of several bishops who were to be his coadjutors, with letters of recommendation and freedom of action in his work.
Source: Grande Munus (Vatican.va)