This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Christianity arrives in Great Moravia through the mission of Constantine, better known by his later monastic name Cyril, and his brother Methodius, sent from Byzantine Thessalonica at the Moravian ruler's own invitation and reaching the kingdom in 863 CE. Their task was not simply to preach but to build a native Slavic language liturgy and clergy, so that the new church could stand on its own rather than depend permanently on Greek or Latin speaking outsiders.
Cyril had already devised the Glagolitic alphabet before the mission began, the first writing system ever created for a Slavic language, and the brothers used it to produce Old Church Slavonic as a liturgical language and to train a body of locally ordained clergy able to serve in the people's own tongue.
The achievement did not hold locally. After Methodius's death in 885, the Slavonic rite clergy he had ordained were expelled from Moravia, and the kingdom itself collapsed under Magyar pressure within a generation, an outcome this account names plainly rather than smoothing over. What survived was the liturgical tradition itself rather than its original home: Old Church Slavonic endured mainly through its later adoption in Bulgaria, carrying the mission's real accomplishment, a Slavic language church, forward into other lands even after Great Moravia's own church fell.