In the early fifth century the Christian world was racked by one of the fiercest theological disputes it had known since the Arian crisis of the previous century. The center of debate turned on the nature of the personhood of Christ, and how divine and human characteristics could combine in Jesus without rendering his subjectivity hopelessly divided, or without reducing his authentic humanness to an insubstantiality. These arguments soon polarized into the conflict between two great churches, Alexandria and Constantinople, and their powerful archbishops, St Cyril (d. 444) and Nestorius (d. ca. 452) respectively. Cyril is, arguably, the most important patristic theologian ever to deal with the issues of Christology. The text here translated is one of his most important and approachable writings, composed in the aftermath of the Council of Ephesus (431) to explain his doctrine to an international audience. He argues here for the single divine presence but fostered and enhanced by it. Accordingly, for St Cyril, Christology becomes a paradigm for the transfigured and redeemed life of the Christian. This book is essential reading for all those interested in the theology and spirituality of the fathers, in the ancient church’s use of scripture, and the way in which the church once creatively expressed its thinking through the media of philosophy and the natural sciences.