After the conquest of Constantinople by the Turks in 1453 John of Segovia, formerly professor at the University of Salamanca and chronicler of the council of Basle, abandoned his scholarly pursuits in order to devote himself exclusively to the study of Islam. This abrupt change decisively influenced the progress of his work on Liber de substancia ecclesie. Shortly before his death on 24 May 1458 John had only finished half of the work accounced in the Prolog. The treatise on the Church, which the Salamancian theologian had intended to write, retains its significance for the development of ecclesiology in the 15th century to this day. This interesting tract outlines an ecclesiastical community of believers within the greater context of salvation history; it also includes a theory of the angels, a theological anthropology and a description of incarnation as a prerequisite for ecclesiology. Thus, Book I deals with the verse “Then war broke out in heaven” (Acts 12.7) of the secret revelation, whereas Book II treats of the mystery of the Church in a christological perspective of the formula “Because of the Church the Son of God became human”. We offer to our readers a work that is rich in its reception of the Holy Bible and characterized by its knowledge of the fathers of the Church and of the teachers of the Middle Ages. This text, edited here for the very first time, reflects the contemporary topics, namely the pervasive frictions within the Church that originated from the conflict between the defenders of papacy and the representatives of a conciliar theory. These historical circumstances have shaped the ecclesiological thinking of John of Segovia, master of the University of Salamanca.