The dissertation deals with the meeting of Eastern and Western Churches at the Council of Ferrara–Florence (1438–39). The focus is on the use and authority of manuscripts: how the material books were part of theological argumentation. The Council was supposed to end the prolonged East–West schism. Four main issues separating the Churches were discussed: Filioque, eucharistic bread, purgatory, and papal primacy. Of these, controversy about the legitimacy of the addition of Filioque to the creed and its orthodoxy took the most space. Arguments were based mostly on authoritative texts originating from the time of the undivided Church. This study shows that not only the commonly accepted texts mattered, but also the material objects, the manuscripts, had to be authoritative. To prove their authority, the Council’s participants adopted various methods.

The materiality of the manuscripts took an important role in the Council. Even before the Council’s opening, manuscripts were searched, collected and studied. It became evident in the Council’s sessions that quoting the authoritative texts by heart was not enough. Manuscripts were needed as physical objects. The texts and their possibly different readings had to be read and analysed. Manuscripts were even loaned and borrowed so that they could be compared with one another. Signs of the manuscript’s history were looked for. The manuscript’s age and writing support were important factors in determining its authenticity and, thus, its authority. What caused debates the most was the corruption found in the leaves of manuscripts. Questions of origin and provenance were also focal, as they could reveal possible mutilation of the text. All these discussions are analysed in this study using the original sources stemming from the Council, the Greek and Latin Acts, Syropoulos’s Memoirs, and correspondence, to name the most important ones.

This study offers a thorough perspective into manuscripts’ role in the Council’s preparations, discussions, and outcomes. The new humanistic methods that were used alongside other argumentative methods, such as scholasticism, not only affected the outcome of the Council but also shaped the individuals and communities that had come to the Council. As the study suggests, this humanistic theology found its arena in the Council, where it could spread.