Cyril, bishop of Alexandria from 412 to 444, is renowned both as one of the most authoritative of all the fathers of the church, and at the same time as one of the most controversial of all church politicians. He oversaw the final extinguishing of pagan religion from Alexandria, and also spent the height of his career as a statesman and an author fighting the doctrines of Nestorius, whose excommunication he brought about at the Council of Ephesus (431). Having spent the first fifteen years of his episcopate writing extensive commentaries on Scripture, from 429 onwards Cyril turned his enormous learning and talent for penning and distributing polemic tracts to the development of doctrinal orthodoxy after he sensed that the new ideas coming out of Constantinople threatenedthe very core of the Christian doctrines of Incarnation and salvation. The three treatises here translated into English for the first time all belong to the period around the ecumenical council.On Orthodoxy to Theodosius was written for the emperor, a year before the council met, with the aim of persuading him that Nestorius's sermons were heretical and that his task as leader of both church and state was to ensure right religious observance. The Defense against the Bishops of Oriens and theDefense against Theodoret were written in the months leading up to the council when Cyril found himself required to defend his notorious Twelve Chapters (or Anathemas), which many bishops in other parts of the empire felt had gone too far in an anti-Nestorian direction. All three works were key parts of Cyril's battle for orthodoxy and mark key moments in the church's progress towards the definition of Christological orthodoxy that was made at Chalcedon.