Pastor aeternus was issued by the First Vatican Council, July 18, 1870. The document defines four doctrines of the Catholic faith: the apostolic primacy conferred on Peter, the perpetuity of the Petrine Primacy in the Roman pontiffs, the definition of the papal primacy as a papal supremacy, and Papal infallibility – infallible teaching authority (magisterium) of the Pope.

The First Ecumenical Council of the Vatican, commonly known as the First Vatican Council or Vatican I, was the 20th ecumenical council of the Catholic Church, held three centuries after the preceding Council of Trent which was adjourned in 1563. The council was convoked by Pope Pius IX on 29 June 1868, under the rising threat of the Kingdom of Italy encroaching on the Papal States. It opened on 8 December 1869 and was adjourned on 20 September 1870 after the Italian Capture of Rome. Its best-known decision is its definition of papal infallibility.