Cyril of Alexandria (AD 370–444) was a towering figure of his time. As the patriarch of Alexandria (in 412), the tradition of leaders before him elevated the Alexandrian see to a position of great influence, rivaled only by Rome, Constantinople, and Antioch. Cyril is most famous for his Christological works, formulating a doctrine of the hypostatic union grounded in Nicene theology that articulates two natures in Christ, a human and divine, which are hypostatically united in the one person [1]. The text of exposition is his work On the Unity of Christ [2]. The treatise is a dialogue whereby Cyril, with theological and philosophical rigor, develops his hypostatic union doctrine. His main opponent is Nestorius, bishop of Constantinople (c. 345), who entered the spotlight for his rejection of the term theotokos (“God bearer”) for the Virgin Mary. He rejected it because he thought no human woman could bear the eternal God. That debate will not be examined here. But it has relevance...