21 The Brothers from Salonika were not only heirs of the faith but also heirs of the culture of Ancient Greece, continued by Byzantium. Everyone knows how important this heritage is for the whole of European culture and, directly or indirectly, for the culture of the entire world. The work of evangelization which they carried out as pioneers in territory inhabited by Slav peoples-contains both a model of what today is called " inculturation the incarnation of the Gospel in native cultures and also the introduction of these cultures into the life of the Church. By incarnating the Gospel in the native culture of the peoples which they were evangelizing, Saints Cyril and Methodius were especially meritorious for the formation and development of that same culture, or rather of many cultures. Indeed all the cultures of the Slav nations owe their "beginning" or development to the work of the Brothers from Salonika. For by their original and ingenious creation of an alphabet for the Slavonic language the Brothers made a fundamental contribution to the culture and literature of all the Slav nations. Furthermore, the translation of the sacred books, carried out by Cyril and Methodius together with their pupils, conferred a capacity and cultural dignity upon the Old Slavonic liturgical language, which became for many hundreds of years not only the ecclesiastical but also the official and literary language, and even the common language of the more educated classes of the greater part of the Slav nations, and in particular of all the Slavs of the Eastern Rite. It was also used in the Church of the Holy Cross in Cracow, where the Slav Benedictines had established themselves. Here were published the first liturgical books printed in this language. Up to the present day this is the language used in the Byzantine liturgy of the Slavonic Eastern Churches of the Rite of Constantinople, both Catholic and Orthodox, in Eastern and South Eastern Europe, as well as in various countries of Western Europe. It is also used in the Roman liturgy of the Catholics of Croatia.
Source: Slavorum Apostoli (Vatican.va)