On the Unity of Christ (Quod Unus Sit Christus)

St. Cyril's late mature dialogue on Christology — a vindication of the one-subject Christology of Ephesus and the Formula of Reunion against the Antiochene exegetes who continued to read the unity of Christ in two-subject terms. The dialogue form, set between Cyril and an unnamed interlocutor (often presumed to be Hermias of Edessa), allows Cyril to gather and refine his christological language in the years between Ephesus (431) and Chalcedon (451). The single most influential summary of Cyril's mature Christology, beloved of both Chalcedonian and non-Chalcedonian traditions.