13 At this point it is an unusual and admirable thing that the holy Brothers, working in such complex and precarious situations, did not seek to impose on the peoples assigned to their preaching either the undeniable superiority of the Greek language and Byzantine culture, or the customs and way of life of the more advanced society in which they had grown up and which necessarily remained familiar and dear to them. Inspired by the ideal of uniting in Christ the new believers, they adapted to the Slavonic language the rich and refined texts of the Byzantine liturgy and likewise adapted to the mentality and customs of the new peoples the subtle and complex elaborations of Greco-Roman law. In following this programme of harmony and peace, Cyril and Methodius were ever respectful of the obligations of their mission. They acknowledged the traditional prerogatives and ecclesiastical rights laid down by Conciliar Canons. Thus, though subjects of the Eastern Empire and believers subject to the Patriarchate of Constantinople, they considered it their duty to give an account of their missionary work to the Roman Pontiff. They likewise submitted to his judgment, in order to obtain his approval, the doctrine which they professed and taught, the liturgical books which they had written in the Slavonic language, and the methods which they were using in evangelizing those peoples. Having undertaken their mission under orders from Constantinople, they then in a sense sought to have it confirmed by approaching the Apostolic See of Rome, the visible center: of the Church's unity. Thus they established the Church with an awareness of her universality as one, holy, catholic and apostolic. This is clearly and explicitly seen in their whole way of acting. It can be said that Jesus' priestly prayer- ut unum sint is their missionary motto in accordance with the Psalmist's words: "Praise the Lord, all nations! Extol him, all peoples". For us today their apostolate also possesses the eloquence of an ecumenical appeal: it is an invitation to restore, in the peace of reconciliation, the unity that was gravely damaged after the time of Cyril and Methodius, and, first and foremost, the unity between East and West. The conviction held by the holy Brothers from Salonika, namely that each local Church is called to enrich with its own endowments the Catholic "pleroma", was in perfect harmony with their evangelical insight that the different conditions of life of the individual Christian Churches can never justify discord, disagreement and divisions in the profession of the one faith and in the exercise of charity.
Source: Slavorum Apostoli (Vatican.va)