The younger, Kyrill, had received a brilliant education at the imperial court, in company with the Emperor Michael, and had been a pupil of the celebrated Photius (afterwards Patriarch), and librarian of St.Sophia, after becoming a monk.

Later on, the brothers had led the life of itinerant missionaries, and had devoted themselves to preaching the Gospel to Jews and Mohammedans.

Thus they were in every way eminently qualified for their new task.
The Slavonians in the Byzantine empire, and the cognate tribes who dwelt nearer the Danube, like the Moravians, had long been in sore need of a Slavonic translation of the Scriptures and the Church books, since they understood neither Greek nor Latin; and for the lack of such a translation many relapsed into heathendom.

Kyrill first busied himself with inventing an alphabet which should accurately reproduce all the varied sounds of the Slavonic tongues.

Tradition asserts that he accomplished this task in the year 855, founding it upon the Greek alphabet, appropriating from the Hebrew, Armenian, and Coptic characters for the sounds which the Greek characters did not represent, and devising new ones for the nasal sounds.