Dei Filius is the incipit of the dogmatic constitution of the First Vatican Council on the Catholic faith, which was adopted unanimously, and issued by Pope Pius IX on 24 April 1870.
The constitution set forth the teaching of "the holy Catholic Apostolic Roman Church" on God, revelation and faith.
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.
The council's main purpose was to clarify Catholic doctrine in response to the rising influence of the modern philosophical trends of the 19th century. In the Dogmatic Constitution on the Catholic Faith (Dei Filius), the council condemned what it considered the errors of rationalism, anarchism, communism, socialism, liberalism, materialism, modernism, naturalism, pantheism, and secularism.