Vatican Council I Called by Pope Pius IX (r. 1846–78) on 8 December 1869 by the bull Aeterni patris, Vatican I was the first Catholic council since Trent, and is counted by Catholics as the twentieth ecumenical council. It was the first attended by a significant number of bishops from outside Europe. Its agenda was initially broad, with extensive preparatory work producing fifty-one schemas (draft documents) for the council fathers to consider. However, the Franco-Prussian war of 1870 forced the proceedings to conclude – without officially closing – after only four sessions, the first of which was merely preparatory. In fact, only two schemas were considered, and, apart from the opening proclamation, only three documents produced.

In their deliberations, the council fathers tended to adhere to one of two perspectives. The majority were ultramontane, supporting the Roman Curia's assertion of the absolute authority of the pope and the Church's rejection of modernity in all its forms (see Ultramontanism). The minority, which included J. H. Newman and many German-speaking bishops, were more liberal. The first document, a profession of faith, stressed conformity and obedience to the Church's teachings as necessary for salvation. The third session produced the dogmatic constitution on the Catholic faith, Dei filius, promulgated on 24 April 1870 as a papal bull. Here the council set out their understanding of the relation between faith and reason in response to the Enlightenment's questioning of the possibility of knowledge of God and the rationality of faith.