11 In order to translate the truths of the Gospel into a new language, they had to make an effort to gain a good grasp of the interior world of those to whom they intended to proclaim the word of God in images and concepts that would sound familiar to them. They realized that an essential condition of the success of their missionary activity was to transpose correctly Biblical notions and Greek theological concepts into a very different context of thought and historical experience. It was a question of a new method of catechesis. To defend its legitimacy and prove its value, Saint Methodius, at first together with his brother and then alone, did not hesitate to answer with docility the invitations to come to Rome, invitations received first from Pope Nicholas I in 867 and then from Pope John VIII in 879. Both Popes wished to compare the doctrine being taught by the Brothers in Greater Moravia with that which the holy Apostles Peter and Paul had passed down, together with the glorious trophy of their holy relics, to the Church's chief episcopal See. Previously, Constantine and his fellow workers had been engaged in creating a new alphabet, so that the truths to be proclaimed and explained could be written in Old Slavonic and would thus be fully comprehended and grasped by their hearers. The effort to learn the language and to understand the mentality of the new peoples to whom they wished to bring the faith was truly worthy of the missionary spirit. Exemplary too was their determination to assimilate and identify themselves with all the needs and expectations of the Slav peoples. Their generous decision to identify themselves with those peoples' life and traditions, once having purified and enlightened them by Revelation, make Cyril and Methodius true models for all the missionaries who in every period have accepted Saint Paul's invitation to become all things to all people in order to redeem all. And in particular for the missionaries who, from ancient times until the present day, from Europe to Asia and today in every continent, have labored to translate the Bible and the texts of the liturgy into the living languages of the various peoples, so as to bring them the one word of God, thus made accessible in each civilization's own forms of expression. Perfect communion in love preserves the Church from all forms of particularism, ethnic exclusivism or racial prejudice, and from any nationalistic arrogance. This communion must elevate and sublimate every purely natural legitimate sentiment of the human heart.
Source: Slavorum Apostoli (Vatican.va)