The claim that the Second Vatican Council erred gravely on doctrine or discipline is contrary to the dogmas of the First Vatican Council, and therefore it is an heretical claim.
The First Vatican Council taught that the Church is indefectible [Vatican I, Pastor Aeternus, chap. 4, n. 2]. The Council cited the teachings of previous Councils — Constantinople (4th), Lyons (2nd), and Florence — in order to explain and reaffirm this dogma, taught by our Lord in Mt 16:18.
The First Vatican Council also taught the dogma that each Roman Pontiff has the charism of truth and a never-failing faith, which is a dogma taught by our Lord in Luke 22:32. [Vatican I, Pastor Aeternus, chap. 4, n. 7]
Now each and every Ecumenical Council, by definition, is a gathering of the body of Bishops led by the Roman Pontiff. And only those decisions of the Council which are also approved by the Pope are formally of the Council. So a Council cannot err gravely on doctrine or discipline, as this would be contrary to the gift of truth given to the Roman Pontiff. For this gift of truth is both a gift of immunity from grave errors at all times and a gift of immunity from all errors when the Pope, or a Council led and approved by the Pope, teaches infallibly. Neither can any Ecumenical Council approved by the Pope teach or promote apostasy, heresy, or schism, as this type of failure of faith would be contrary to the gift of a never-failing faith given to the Roman Pontiff.
I argue, based on the last part of Luke 22:32, that the body of Bishops, only as a body not as individuals, also has these gifts of truth and a never-failing faith. But that point is a theological conclusion of mine. The dogma of the First Vatican Council is no mere theological opinion, but a required belief under pain of heresy: every Roman Pontiff has the charism of truth and a never-failing faith.
Therefore, the Second Vatican Council could not have erred gravely on doctrine or discipline, as the teachings and decisions of that Council were approved by Pope Saint Paul VI, as well as by the subsequent Roman Pontiffs. If Vatican II erred gravely, then the Popes would have erred gravely in formally approving of the documents of the Council, as Pope Saint Paul VI did, and in continuing to teach the Church from those documents. Such grave errors are impossible for any Roman Pontiff, because no human person can resist the prevenient grace of God and because the promise and prayer of our Lord (Lk 22:32) cannot fail.
Moreover, if anyone were to claim that the Second Vatican Council taught heresy, or created a schism, or taught, promoted, or initiated apostasy or idolatry, such claims would be contrary to the dogma that the Pope has a never-failing faith. For the decisions of a Council are not formally of the Council unless the Pope approves, and the subsequent Popes have continued approving and teaching from the works of that Council.
Thus, it is the grave sin of heresy to accuse the Second Vatican Council of any grave errors or of any failures of faith. For Councils are led and approved by the Roman Pontiffs, who have the charism of truth and never-failing faith. The dogmas of Vatican I, therefore, confirm that the teachings and decisions of Vatican II cannot have erred gravely and cannot represent any kind of failure of faith. Those who reject Vatican II are thereby rejecting the dogmas of Vatican I, and so they are heretics.
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