Byzantine Greek: The Language of Orthodox Christian Theology

Byzantine Greek, also called Medieval Greek, covers the form of the Greek language used across the Byzantine Empire from roughly the fourth or fifth century through the empire's fall to the Ottomans in 1453, developing from Koine Greek while incorporating grammatical simplification and vocabulary change over its roughly thousand-year span, and belonging, like all historical Greek registers, to the Hellenic branch of the Indo-European family. Byzantine Greek functioned as the primary language of the Eastern Roman Empire's imperial administration, its highly educated literary elite, and crucially its Orthodox Christian Church, whose major theological councils, including Chalcedon in 451 and the later iconoclasm controversies of the eighth and ninth centuries, were conducted and their conclusions recorded substantially in this evolving form of Greek.

The Byzantine liturgical and theological tradition, including the Divine Liturgy of St John Chrysostom still used across Eastern Orthodoxy today, and the extensive corpus of Byzantine patristic and monastic writing, including major figures such as Maximus the Confessor and John of Damascus, represent one of Christianity's largest bodies of theological literature in any single historical language register.