Cyril was a scholarly archbishop and a prolific writer. In the early years of his active life in the Church he wrote several exegetical documents. Among these were: Commentaries on the Old Testament, Thesaurus, Discourse Against Arians, Commentary on St. John's Gospel, and Dialogues on the Trinity. In 429 as the Christological controversies increased, the output of his writings was so extensive that his opponents could not match it. His writings and his theology have remained central to the tradition of the Fathers and to all Orthodox to this day. (from Wikipedia)Book I of Commentary on St John's Gospel covers John 1:1-28.