Pastor aeternus declares papal infallibility

Background. Pius IX convoked the First Vatican Council on 29 June 1868, under the rising threat of the Kingdom of Italy encroaching on the Papal States. The council met in 1869–1870, where it defined the dogma of papal infallibility in the document Pastor aeternus. Before 1870, belief in papal infallibility was not a defined requirement of Catholic faith, though the doctrine is claimed to have existed in medieval theology and to have been the majority opinion at the time of the Counter-Reformation.

Definition of Infallibility. The dogma defines the Bishop of Rome as possessing infallibility when exercising his office as shepherd and teacher of all Christians. This authority is exercised in virtue of supreme apostolic authority when defining a doctrine concerning faith or morals to be held by the whole Church. The definition was seen not as a creation of the Church, but as the dogmatic definition of a truth about the Church Magisterium.

Opposition and Aftermath. The definition raised considerable opposition in liberal theological circles in the Netherlands, Austria, Germany, and Switzerland. Ignaz von Döllinger was excommunicated in 1871 for refusing to accept the council's decision. Bismarck feared the international Catholic Church would use the papal claim of infallibility to control national Germany. Dissent arose among some Catholics, almost exclusively German, Austrian, and Swiss.