ON THE INFALLIBLE TEACHING
OF THE ROMAN PONTIFF
[A summary of Pastor Aeternus up to this point: first, in the introduction and first chapter, it was emphasized that Christ established in St. Peter a true primacy of jurisdiction over the Church. In the second chapter it was established that this primacy was sustained in the occupants of the Holy See, i.e. the Roman Pontiffs. In the third chapter the specific qualities of the universal jurisdiction of the Roman Pontiffs were analyzed: its supremacy, universality, and immediacy. The final chapter of the constitution is the best known, not only of Pastor Aeternus, but of the work of Vatican I as a whole. The subject is the charism of infallibility granted to the popes. I invite the reader to read this chapter through on his own. I will refrain from commenting on it as I have the earlier chapters, and instead embolden certain passages I find especially noteworthy.]
Moreover, this Holy See has always held, the perpetual practice of the Church confirms, and Ecumenical Councils also have declared -- especially those in which the East with the West met in the union of faith and charity -- that the supreme power of teaching is also included in the Apostolic primacy which the Roman Pontiff, as the successor of Peter, Prince of the Apostles, possesses over the whole Church. For the Fathers of the Fourth Council of Constantinople, following in the footsteps of their predecessors, gave forth this solemn profession: The first condition of salvation is to keep the rule of the true faith.
And because the sentence of our Lord Jesus Christ cannot be passed over, who said: "You are Peter, and upon this rock I will build my Church" (Mt 16:18), these things which have been said are approved by events, because in the Apostolic See the Catholic Religion and her holy and well-known doctrine has always been kept undefiled. Desiring, therefore, not, to be in the least degree separated from the faith and doctrine of that See, we hope that we may deserve to be in the one communion, which the Apostolic See preaches, in which is the entire and true solidity of the Christian religion. 
And, with the approval of the Second Council of Lyons, the Greeks professed that the Holy Roman Church enjoys supreme and full Primacy and preeminence over the whole Catholic Church, which She truly and humbly acknowledges that She has received with the plenitude of power from our Lord Himself in the person of blessed Peter, Prince or Head of the Apostles, whose successor is the Roman Pontiff; and as the Apostolic See is bound before all others to defend the truth of faith, so also if any questions regarding faith shall arise, they must be defined by its judgment.
Finally, the Council of Florence defined: That the Roman Pontiff is the true Vicar of Christ, and the Head of the whole Church, and the Father and Teacher of all Christians; and that to him in blessed Peter was delivered by our Lord Jesus Christ the full power of pasturing, ruling, and governing the whole Church. (John 21:15-17).
To satisfy this pastoral duty, our predecessors ever made unwearied efforts that the salutary doctrine of Christ might be propagated among all the nations of the earth, and, with equal care, they watched that it might be preserved, genuine and pure, where it had been received. Therefore, the Bishops of the whole world, at times individually, at times assembled in a synod, following the long-established custom of the Churches, and the form of the ancient rule, sent word to this Apostolic See especially of those dangers which sprang up in matters of faith, that the losses of faith might be most effectually repaired there, where the faith cannot fail.
And the Roman Pontiffs, according to the exigencies of times and circumstances, sometimes assembling Ecumenical Councils, or asking for the mind of the Church scattered throughout the world, sometimes by particular Synods, sometimes using other helps which Divine Providence supplied, defined as to be held those things which, with the help of God, they had recognized as conformable with the Sacred Scriptures and Apostolic Traditions. For the Holy Spirit was not promised to the successors of Peter that by His revelation they might make known new doctrine, but that by His assistance they might inviolably keep and faithfully expound the Revelation, the Deposit of Faith, delivered through the Apostles.
And indeed, all the venerable Fathers have embraced, and the holy orthodox Doctors have venerated and followed, their Apostolic doctrine; knowing most fully that this See of holy Peter remains ever free from all blemish of error, according to the Divine promise that the Lord our Savior made to the Prince of His disciples: "But I have prayed for you, so that your faith may not fail, and so that you, once converted, may confirm your brothers." (Lk 22:32).
This gift, then, of truth and never-failing faith was conferred by heaven upon Peter and his successors in this Chair, that they might perform their high office for the salvation of all; that the whole flock of Christ, kept away from the poisonous food of error by them, might be nourished with the pasture of heavenly doctrine; that the occasion of schism being removed, the whole Church might be kept one, and, resting on its foundation, might stand firm against the gates of Hell.
But since, in this very age in which the salutary efficacy of the Apostolic office is most of all required, not a few are found who take away from its authority, We judge it altogether necessary to assert solemnly the prerogative which the only-begotten Son of God found worthy to join with the supreme pastoral office.
Therefore, faithfully adhering to the tradition received from the beginning of the Christian faith, for the glory of God Our Savior, the exaltation of the Catholic Religion, and the salvation of Christian people, the Sacred Council approving, We teach and define that it is a divinely-revealed dogma: that the Roman Pontiff, when he speaks ex Cathedra, that is, when in discharge of the office of Pastor and Teacher 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 for defining doctrine regarding faith or morals: and that therefore such definitions of the Roman Pontiff are irreformable of themselves, and not from the consent of the Church.
But if anyone -- God forbid -- should presume to contradict this Our definition; let him be anathema.
Given at Rome in Public Session solemnly held in the Vatican Basilica in the year of Our Lord one thousand eight hundred and seventy, on the eighteenth day of July, in the twenty-fifth year of our Pontificate.