In 862 the Slavic Prince Rostislav of Moravia sent a request to Constantinople for missionaries who could help him strengthen Christianity among the Slavs. (It must be stated that the motives impelling him were not solely religious, but also political. By strengthening Christianity among the Slavs he was fortifying his own national independence against the new historical colossus of Germanism, which was forming at the time). The choice, probably made with the help of Patriarch Photius, naturally fell on the two brothers, and in the middle of 864 they arrived in Velehrad, the capital of Moravia.

Alexander Schmemann, A History of the Orthodox Church

Alexander Schmemann, A History of the Orthodox Church

Here, along with their purely missionary activity, they began their great work of translating Christian writings into Slavic. From a legalistic point of view, they were working in a region that had been part of the sphere of influence of the Roman Church from ancient times.

These were years of intense struggle between Pope Nicholas I and Patriarch Photius. To regularize their situation, or perhaps because of a summons from Rome, which had become disturbed at the growth of Greek influence among the western Slavs, the brothers quickly went to the Western capital, where Nicholas’ successor Hadrian greeted them ceremoniously and affectionately. The Slavic Gospel was placed on the altar of St. Mary as a sign of papal blessing, and the Slavic liturgy was performed in many Roman Churches.

The mission to the Slavs, beginning under the dual blessing of Byzantium and Rome, promised quick success, but the story of Constantine records the beginning of opposition as well, against which the brothers were obliged to defend their work. He was not destined to return to Moravia but died in Rome in 869, becoming a monk just before his death and obtaining his brother’s promise to continue the work he had begun.