What remains historically from the First Vatican Council (1869-1870) is, first and foremost, the dogma of Papal Infallibility. The council’s prehistory seems to be forgotten; its consequences are easily overlooked or often interpreted in a very one-sided way. In addition, in a world that is becoming more and more secular, authority, and therefore also papal authority, is being questioned inside and outside the Church. Therefore, this elective course on the First Vatican Council aims to cover its pre-historical and theological context, its course and progress, its ecclesial aftermath, and its theological consequences so that students are able to understand the significance of the First Vatican Council for today. The lectures focus mainly on these eight aspects: 1. Early Christianity of the West granted the Bishop of Rome a special position. Thereby, over the centuries, Papal Primacy developed. In theological terms, a sense of Papal Infallibility prevailed. It enjoyed widespread acceptance until the 18th century. 2. From the outside, the Church as an institution was then increasingly challenged by philosophical enlightenment (liberalism, skepticism, relativism...) and growing political nationalisms. Revolutions in many countries threatened the Church and its members, including the papacy, even physically. Pius IV died in French exile in 1799. 3. Within the Church, movements like Conciliarism, Gallicanism, Febronianism, Josephinism, and Jansenism strengthened bishop-centered ecclesiologies, redefining papal authority. 4. As a reaction, in the 19th century, the international movement of pope-centered Ultramontanism developed inner resistance. The personality of Pius IX (1792-1878) now stood for ecclesiastical uncompromising ecclesiastical leadership. In 1864, [no comma] the encyclical letter Quanta cura and its appendix Syllabus of Errors were landmark documents. It was in this spirit that Pius IX convened “his” Council. 5. Despite all justified criticism, Pius IX made the First Vatican Council the most modern international gathering of its time. The Council freed Catholic bishops from all over the world from their provincialism and nationalism, made use of modern print media for the first time (Civilta Cattolica...), and relied strongly on the input of conservative Catholic laymen (Maistre, Veuillot, Goerrs...). 6. The weakness of Vatican I, as identified by the liberal minority of Council fathers, lies not in the definition but in the inadequate interpretation and explanation of Papal Infallibility. The minority bishops sensed the imbalance of holy Scripture, tradition, pastoral care, and law. Problems identified by the minority were later forcefully revealed in the modernist crisis. The sole act of definition, provoked by Pius IX and “his” ultramontane majority of bishops, did not replace argumentation. 7. The maneuvering of Pius IX and of the leaders of both the majority and the minority was not without effect. Partisanship made the Council appear “unfree.” The division between the bishops, which became clear at the end of the council, is a tragedy. However, there was no major schism. The Franco-Prussian War (1870-1871) prevented the schismatic group of “Old Catholics” from spreading. The spiritual greatness of Vatican I, however, lies in the fact that the minority later surrendered to the definition and its implementation. History was to show that in the coming epoch of the 20th century, marked by two World Wars, only an ultramontane church was able to survive. 8. Only the Second Vatican Council (1962-1965) would theoretically free the Church from the ultramontane and clerical ghetto. This awakening is still in progress. Since 1963, all the pontificates bear witness to this fact. The Synodality of the Church, emphasized by Pope Francis (2013-2025) since 2019, is a further step in the conciliar awakening that connects and challenges Catholic life with the realities of the secular world.