15 Venerable brothers, the memory of these events causes Us great joy. We are deeply moved by the magnificent association of the Slavic nation and the Roman church, an association with the noblest beginnings. Though these two apostles of the Christian faith went from Constantinople to preach to people in foreign lands, it was from this Apostolic See, the center of Catholic unity, that they received the investiture of their mission or, as happened more than once, its solemn approbation. Truly it was here in this city of Rome that they rendered an account of their mission and answered their accusers. It was here at the tombs of Peter and Paul that they swore to keep the Catholic faith. It was here that they received episcopal consecration and the power to establish the sacred hierarchy, while observing in it the distinction of each order. Finally, it was here that they sought and obtained permission to use the Slavic language in holy rites. This year, ten centuries will have passed since Pope John VIII wrote to prince Svatopluk of Moravia: "It is right that we praise the Slavic language, which re-echoes with the praises due to God. We ordain that the proclamations and works of our Lord Jesus Christ should be said in that language. Nothing in true faith, or doctrine forbids us to sing the Mass in the Slavic language, or to read the Gospel lessons (correctly translated and interpreted) in it, or to chant in it the Divine Hours." After many changes, Pope Benedict XIV sanctioned this custom in an apostolic letter dated August 25, 1754. Whenever the rulers of the peoples evangelized by Cyril and Methodius asked them, the popes gave generously of their humanity in teaching, kindness in giving advice, and singular good will wherever possible. Above all, Rastislav, Svatopluk, Kocel, Saint Ludmilla, and Boris have experienced the remarkable love of Our predecessors.
Source: Grande Munus (Vatican.va)