The Cistercian Hermann Zoest’s Treatise on Leavened and Unleavened Bread (‘De fermento et azimo’): Oecumenism, Exegesis, and Science at the Council of Basel

This critical edition of the Cistercian astronomer and conciliarist Hermann Zoest of Munster’s De fermento et azimo, surviving in a dozen complete manuscripts, makes available the greatest medieval treatise concerning the type of bread that Jesus broke at the Last Supper. Since the so-called Schism of 1054, the Greeks, who employed ordinary leavened bread in the sacrament of the Eucharist, routinely claimed that the Latin use of unleavened bread was invalid and did not involve the Body of Christ. Hermann composed his treatise in 1436 at the Council of Basel, with the oecumenical goal of facilitating Church Union. Relying on astronomy, biblical exegesis, conversation with Greeks, and, in a later revision, information from the famous Jewish convert Bishop Paul of Burgos, Hermann came to the conclusion that the Last Supper occurred before Passover when the Jews were still eating leavened bread, although he allowed for the possibility that Jesus established a new rite with unleavened bread. After enumerating the disagreements between Greeks and Latins, Hermann advised that they focus on the faith and ignore what he labelled ceremonial differences.