Twentieth Ecumenical (Dogmatic) Council, Vatican I (1869–1870)
“Neque enim Petri successoribus Spiritus sanctus promissus est, ut eo revelante novam doctrinam patefacerent, sed ut eo assistente traditam per apostolos revelationem seu fidei depositum sancte custodirent et fideliter exponerent.” (Constitutio Dogmatica Prima de Ecclesia Christi [Pastor Aeternus], cap. 4, “De Romani Pontificis Infallibili Magisterio”)
[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.]
Address at the First Vatican Council by Archbishop Purcell of Cincinnati, Ohio, on the infallibility of the Pope as defined at the Council.
The question was also raised by a Cardinal: “What is to be done with the Pope if he becomes a heretic?”
It was answered that:
“There has never been such a case; the Council of Bishops could depose him for heresy, for from the moment he becomes a heretic he is not the head or even a member of the Church. The Church would not be, for a moment, obliged to listen to him when he begins to teach a doctrine the Church knows to be a false doctrine, and he would cease to be Pope, being deposed by God Himself.
If the Pope, for instance, were to say that the belief in God is false, you would not be obliged to believe him, or if he were to deny the rest of the creed, ‘I believe in Christ,’ etc. The supposition is injurious to the Holy Father in the very idea, but serves to show you the fullness with which the subject has been considered and the ample thought given to every possibility. If he denies any dogma of the Church held by every true believer, he is no more Pope than either you or I; and so in this respect the dogma of infallibility amounts to nothing as an article of temporal government or cover for heresy.”
In drafting the definition of the Dogma of Infallibility in 1869, the theological experts (periti) of Vatican Council I actually discovered that more than forty popes had preached personal doctrinal errors in preceding centuries, though not in an infallible context.
The Council Fathers, having reaffirmed what the Church had always taught—that it is necessary for salvation to be in union with the Bishop of Rome and that whoever rejects his authority cannot hope to be saved—went on to reason that the Pope therefore could not err or lead his flock astray. Otherwise, the faithful might, on certain occasions, find themselves in the position of having to follow him into error. Since no one is ever bound to commit an evil act, this would be an absurdity.
At this point the Council also had to define the limits of infallibility and lay down the precise conditions that must be met for a pronouncement to be ex cathedra. Clearly, the Council was aware that obedience to the Pope—who is only relatively infallible—could not under all circumstances be identified with obedience to God, Who alone is the Source of all Truth and Holiness. Not only was the Infallibility of the Pope defined at the Vatican Council, but also the limits and extent of this infallibility.
To put it another way, the Council also established that outside these limits the Pope remains capable of erring and is not entitled to command blind obedience.
Source:
The quotation from Archbishop Purcell comes from an address he delivered in Cincinnati, Ohio, after returning from the First Vatican Council. It summarizes a discussion that reportedly took place among the Council Fathers.
The most commonly cited reference for this exact wording is:
Rev. James J. McGovern, Life and Life Work of Pope Leo XIII (Chicago: Allied Printing, 1903), pages 236–241 (especially p. 241).