More evidence that, ultimately, the Pope is the rule of faith:

St Peter’s successors = rule of faith

My Enemy Colin,

I should have been more precise.

When the legitimate successor of St. Peter teaches the whole Church a doctrine concerning matters of faith and morals, that is the rule of faith.

If only the fathers of the Vatican Council I had you as adviser to improve the accuracy of their definition, Colin.

If you disagree with that statement, you have to necessarily say that the Church’s Magisterium is capable of teaching pernicious error to the whole Church, which is absurd.

• For the holy Spirit was promised to the successors of Peter
  
  • not so that they might, by his revelation, make known some new doctrine,
  • but that, by his assistance, they might religiously guard and faithfully expound the revelation or deposit of faith transmitted by the apostles.
  Indeed, their apostolic teaching was
  
  • embraced by all the venerable fathers and
  • reverenced and followed by all the holy orthodox doctors,
  for they knew very well that this see of St. Peter always remains unblemished by any error, in accordance with the divine promise of our Lord and Saviour to the prince of his disciples: I have prayed for you that your faith may not fail; and when you have turned again, strengthen your brethren [60] .
• This gift of truth and never-failing faith was therefore divinely conferred on Peter and his successors in this see so that they might discharge their exalted office for the salvation of all, and so that the whole flock of Christ might be kept away by them from the poisonous food of error and be nourished with the sustenance of heavenly doctrine. Thus the tendency to schism is removed and the whole church is preserved in unity, and, resting on its foundation, can stand firm against the gates of hell.
• But since in this very age when the salutary effectiveness of the apostolic office is most especially needed, not a few are to be found who disparage its authority, we judge it absolutely necessary to affirm solemnly the prerogative which the only-begotten Son of God was pleased to attach to the supreme pastoral office.
• Therefore,
  
  • faithfully adhering to the tradition received from the beginning of the christian faith,
  • to the glory of God our saviour,
  • for the exaltation of the catholic religion and
  • for the salvation of the christian people,
  • with the approval of the sacred council,
    

    ---

  • we teach and define as a divinely revealed dogma that
    
    • when the Roman pontiff speaks EX CATHEDRA,
      
      • that is, when,
      • in the exercise of his office as shepherd and teacher of all Christians,
      • in virtue of his supreme apostolic authority,
      • he defines a doctrine concerning faith or morals to be held by the whole church,
    • he possesses,
      
      • by the divine assistance promised to him in blessed Peter,
    • that infallibility which the divine Redeemer willed his church to enjoy in defining doctrine concerning faith or morals.
    • Therefore, such definitions of the Roman pontiff are of themselves, and not by 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.

That is false. Vatican I does not admit of interpretation, it defined very clearly when the Pope is infallible, under what conditions Peter enjoys that charism of 'never failing faith'.

For which cause, this council has thought good, that the Symbol of faith which the holy Roman Church makes use of,–as being that principle wherein all who profess the faith of Christ necessarily agree, and that firm and alone foundation against which the gates of hell shall never prevail,–be expressed in the very same words in which it is read in all the churches. Which Symbol is as follows:

Council of Trent, Third Session, Decree Touching the Symbol of Faith

          I believe in one God, the Father Almighty, maker of heaven and earth,
          of all things visible and invisible; and in one Lord Jesus Christ,
          the only-begotten Son of God, and born of the Father before all ages;
          God of God, light of light, true God of true God; begotten, not made,
          consubstantial with the Father, by whom all things were made:
          who for us men, and for our salvation, came down from the heavens,
          and was incarnate by the Holy Ghost of the Virgin Mary, and was made man:
          crucified also for us under Pontius Pilate, he suffered and was buried;
          and he rose again on the third day, according to the Scriptures;
          and he ascended into heaven, sitteth at the right hand of the Father ;
          and again he will come with glory to judge the living and the dead;
          of whose kingdom there shall be no end: and in the Holy Ghost the Lord,
          and the giver of life, who proceedeth from the Father and the Son;
          who with the Father and the Son together is adored and glorified;
          who spoke by the prophets and one holy Catholic and Apostolic Church.

          I confess one baptism for the remission of sins;
          and I look for the resurrection of the dead,
          and the life of the world to come.

          Amen.

https://www.papalencyclicals.net/councils/trent/third-session.htm