Papal infallibility: The Dogmatic definition of 1870
In the conclusion of the fourth chapter of its Dogmatic Constitution on the Church Pastor Aeternus, solemnly promulgated by Pope Pius IX, the First Vatican Council in 1870 declared the following:
“We teach and define that it is a dogma Divinely revealed that the Roman pontiff when he speaks ex cathedra, that is when in discharge of the office of pastor and doctor of all Christians, by virtue of his supreme Apostolic authority, he defines a doctrine regarding faith or morals to be held by the universal Church, by the Divine assistance promised to him in Blessed Peter, is possessed of that infallibility with which the Divine Redeemer willed that his Church should be endowed in defining doctrine regarding faith or morals, and that therefore such definitions of the Roman pontiff are of themselves and not from the consent of the Church irreformable.
"So then, should anyone, which God forbid, have the temerity to reject this definition of ours: let him be anathema.”
(Vatican Council, Sess. IV , Const. de Ecclesiâ Christi, Chapter iv.)