20 The message of the Gospel which Saints Cyril and Methodius translated for the Slav peoples, drawing with wisdom from the treasury of the Church "things old and new", was transmitted through preaching and instruction in accordance with the eternal truths, at the same time being adapted to the concrete historical situation. Thanks to the missionary efforts of both Saints, the Slav peoples were able for the first time to realize their own vocation to share in the eternal design of the Most Holy Trinity, in the universal plan for the salvation of the world. At the same time, they can recognized their role at the service of the whole history of the humanity created by God the Father, redeemed by the Son our Savior and enlightened by the Holy Spirit. Thanks to this preaching, duly approved by the authorities of the Church-the Bishops of Rome and the Patriarchs of Constantinople-the Slavs were able to feel that they too, together with the other nations of the earth, were descendants and heirs of the promise made by God to Abraham. In this way, thanks to the ecclesiastical organization created by Saint Methodius and thanks to their awareness of their own Christian identity, the Slavs took their destined place in the Church which had now arisen also in that part of Europe. For this reason, their modern descendants keep in grateful and everlasting remembrance the one who became the link that binds them to the chain of the great heralds of the divine Revelation of the Old and New Testaments: "After all of these, the merciful God, in our own time, raised up for the good work, for the sake of our own people, for whom nobody had ever cared, our Teacher, the holy Methodius, whose virtues and struggles we unblushingly compare, one by one, to those of these men pleasing to God".
Source: Slavorum Apostoli (Vatican.va)