Cyril of Thessalonica

He was born Constantine at Thessalonica around 826, in a city sitting on the edge of a Slavic-speaking hinterland, which is the usual explanation for how two Greek brothers came to know Slavic well enough to translate into it. His father held a military office in the city. The younger son went to Constantinople for his education, studied in the circle around Photios, and took a post as librarian at Hagia Sophia and then as a teacher of philosophy. The epithet the Philosopher is contemporary, and it stayed attached to him.

Before Moravia he was twice sent abroad on the empire's business, first towards the Arabs and then, around 860, to the Khazars north of the Caucasus, on both occasions to argue the Christian case in a formal disputation. The Khazar journey produced no conversion, but it did produce a find. At Cherson in the Crimea he identified relics as those of Clement of Rome, and he carried them with him from then on. They were eventually deposited in Rome, and they are part of why the brothers were received there as well as they were.

In 862 Prince Rastislav of Moravia asked Constantinople for teachers who could instruct his people in their own tongue, wanting a church that did not arrive with Frankish clergy attached. The emperor sent the brothers. The request was political and the answer was political too, and what came out of it was a writing system and a literature.

The Glagolitic alphabet is an invention rather than an adaptation. Its letters are not Greek letters with additions but an original set, and its inventory was built to the sounds of ninth-century Slavic instead of inherited from a language that lacked several of them. That is rare. Most scripts are borrowed and then patched. This one was designed, and designed for a particular job, which was to carry the Gospels and the liturgy.

The translation work began before the brothers left for Moravia and continued there. What came out of it is the language now called Old Church Slavonic, the first literary Slavic language, and it became the liturgical language of a large part of the Orthodox world.

The opposition came from the Frankish clergy already working in the region, and it was doctrinal as well as territorial. The argument, which later writers call the trilingual heresy, held that scripture and worship belonged properly to Hebrew, Greek and Latin, the three languages of the inscription on the cross, and to no other. Constantine argued against it at Venice and then at Rome, where Pope Hadrian II approved the Slavonic books and had them laid on an altar in the city. He did not leave Rome again. He took monastic vows and the name Cyril shortly before his death there on 14 February 869, and he was buried in the basilica of San Clemente.