1 The great duty of spreading the Christian name was entrusted in a special way to Peter, the head of the apostles, and to his successors. It urges the popes to send messengers of the Gospel to the various peoples of the earth, as the affairs of the merciful God demand. For this reason they sent Augustine to the Britons, Patrick to the Irish, Boniface to the Germans, and Willebrord to the Flemish, Dutch, and Belgians. Often they sent other men to other peoples to care for their souls. So in the exercise of their apostolic duty, they commissioned the holy men Cyril and Methodius to go to the Slavic people. Through their presence and more especially through their labors, those people have seen the light of the Gospel and have been led from their barbarian ways to a humane and civilized culture.
Source: Grande Munus (Vatican.va)