Papal Indefectibility is based on the words of Jesus in Luke 22:32 and on the ancient constant teaching of the Church on the role of the Roman Pontiff.

{22:32} But I have prayed for you, so that your faith may not fail, and so that you, once converted, may confirm your brothers.”

The faith of Peter and his successors will never fail, just as the Church has always taught.

Pope Saint Leo I: “The order of truth remains; blessed Peter, keeping the strength of the rock, does not abandon the helm of the Church. Whatever We do rightly is his work, whose power lives in his See…. In the person of My lowliness he is seen, he is honored, in whom remains the care of all pastors and of the sheep of their charge. His power does not fail, even in an unworthy heir.”

Pope Pelagius II, 590 AD, writing to the Bishops of Istria: “For you know how the Lord in the Gospel declares: ‘Simon, Simon, behold Satan has desired you that he might sift you as wheat, but I have prayed to the Father for thee, that thy faith fail not, and thou, being converted, confirm thy brethren.’ See, beloved, the truth cannot be falsified, nor can the faith of Peter ever be shaken or changed.”

Pope Saint Agatho: “For this is the rule of the true faith, which this spiritual mother of your most tranquil empire, the Apostolic Church of Christ, has both in prosperity and in adversity always held and defended with energy; which, it will be proved, by the grace of Almighty God, has never erred from the path of the apostolic tradition, nor has she been depraved by yielding to heretical innovations, but from the beginning she has received the Christian faith from her founders, the princes of the Apostles of Christ, and remains undefiled unto the end, according to the divine promise of the Lord and Savior himself, which he uttered in the holy Gospels to the prince of his disciples: saying, ‘Peter, Peter, behold, Satan hath desired to have you, that he might sift you as wheat; but I have prayed for thee, that (thy) faith fail not. And when thou art converted, strengthen thy brethren.’ Let your tranquil Clemency therefore consider, since it is the Lord and Savior of all, whose faith it is, that promised that Peter’s faith should not fail and exhorted him to strengthen his brethren, how it is known to all that the Apostolic pontiffs, the predecessors of my littleness, have always confidently done this very thing: of whom also our littleness, since I have received this ministry by divine designation, wishes to be the follower, although unequal to them and the least of all.”

Pope Saint Leo IX: “Without a doubt, it was for him alone, whom the Lord and Savior asserted that he prayed that his faith would not fail, saying, ‘I have prayed for thee, etc.’ [Lk 22:32]. Such a venerable and efficacious prayer has obtained that to this point the faith of Peter has not failed, nor can it be believed that it is ever going to fail in his throne.”

Pope Saint Leo IX: “By the See of the Chief of the Apostles, namely by the Roman Church, through the same Peter, as well as through his successors, have not the comments of all the heretics been disapproved, rejected, and overcome, and the hearts of the brethren in the faith of Peter — which so far neither has failed, nor up to the end will fail — been strengthened?”

Francisco Suarez: “The faith of Peter was catholic and not able to fail; but the faith of the Roman Church is the faith of Peter; therefore, the faith of the Roman Church is the catholic faith, from which this See can never defect.”

Origen, 184-253: “It is manifest, even if it were not expressed, because the gates of Hell can prevail against neither Peter, nor the Church, for if they prevailed against the rock on which the Church was founded, they would prevail against the Church.” [Mt 16:18]

Origen, 184-253: “neither against the rock upon which Christ builds His Church, nor against the Church, shall the gates of Hell prevail.” [quoted with agreement by Pope Leo XIII in Satis Cognitum]

Saint Cyprian, Bishop, 210-258: “Where Peter is, there is the Church”, repeated by Saint Ambrose and Saint Boniface. Confirmed by Pope Benedict XV in the encyclical In Hac Tanta.

What Indefectibility Means

Papal infallibility extends only to that subset of papal teachings which meet all of the criteria for an infallible papal teaching. Any other papal teachings, short of that full set of criteria (taught by Vatican I and reiterated by Vatican II), would be non-infallible and subject to the possibility of error.

Papal indefectibility extends to the person of the Roman Pontiff, that his faith will not fail, and to the exercise of the Keys of Peter by the Roman Pontiff, that his non-infallible teachings and decisions of discipline will not err gravely and will not lead the faithful away from the true Church and the true path of faith, morals, and salvation taught by Jesus Christ. Errors are still possible, but these errors can never rise to the extent of harming the indefectibility of the Church, nor of defection from the true faith by the Roman Pontiff, the See of Peter, the body of Bishops, or the body of the faithful.

* The Pope can never fall into apostasy, heresy, or schism.
* The Pope can never lose his office, by a failure of faith, and so become a false Pope or antipope.
* A heretic elected to the office of Roman Pontiff (e.g. Vigilius 537-555) will be converted by the prevenient grace of God, and so become a rock of faith instead of a heretic.
* Every non-infallible teaching of the Roman Pontiff under the Keys of Peter is free from every grave error, but not necessarily free from all error.
* The Pope can err in his personal opinions, to a greater extent, but never to the extent of apostasy, heresy, or schism as the person of the Roman Pontiff, as well as his exercise of the office, has a never-failing faith.
* Since the indefectible Roman Pontiff confirms his brother Bishops in the faith, the body of Bishops (as a body, not individually) is also preserved from grave error in doctrine and discipline and from defecting away from the faith, as a body, by apostasy, heresy, or schism.
* The indefectible Roman Pontiff also confirms the faithful in the faith, as is clear from the threefold commission of Jesus to Peter to feed his sheep and lambs. Therefore, the faithful, as a body, cannot defect from the faith by apostasy, heresy, or schism.

Congregation for the Doctrine of the Faith, Cardinal Levada: “The Holy Father, Pope Benedict XVI, in an Audience granted to the Prefect of the Congregation for the Doctrine of the Faith, Cardinal William Joseph Levada, on January 14, 2011, approved the decisions of the Ordinary Session of the Congregation, and ordered their implementation. This action by the Holy Father should be understood in virtue of the mandate given by the Lord to Simon Peter as the rock on which He founded his Church (cf. Luke 22:32): ‘I have prayed for you, Peter, that your faith may not fail; and when you have turned to me, you must strengthen the faith of your brothers and sisters.’ This Scripture passage has long been applied to the role of the Successors of Peter as Head of the Apostolic College of Bishops; it also applies to the role of the Pope as Chief Shepherd and Pastor of the Universal Church.”

What Indefectibility Does NOT Mean

* indefectibility does not mean that the Pope can never err
* indefectibility does not mean that the Pope can never sin gravely
* indefectibility does not mean that we cannot disagree with a non-infallible papal decision of doctrine or discipline; the extent of faithful disagreement that is possible reaches to the extent of the possibility of error.

More Reading:
* Perennial Catholic Teaching on the Roman Pontiff
* Freedom from Grave Error in the Apostolic See

Ronald L Conte Jr