Just a bit of history for the discussion.
Pastor Aeternus relies sparingly on the testimony of the Church Fathers to uphold its definition of the extraordinary papal magisterium. Gasser nonetheless referred to the support they provided for the council’s teaching on papal infallibility. They are, so to speak, in the background of the definiton. From the council Father’s interventions in this regard, it seems that they believed that the pope’s primacy of teaching had been accepted without contestation until the appearance of conciliarism in the fourteenth century. The Majority held that the collective testimony of the Church Fathers confirmed not only the pope’s primacy of jurisdiction but also his primacy of teaching. While contemporary scholars are more cautious about detecting irrefutable proofs in the patristic sources, these venerable texts still provide valuable testimony about Rome’s early teaching authority. The writers of the ancient Church regarded communion with the see of Peter as a reliable guarantee for knowing the truth of the apostolic faith.
According to the council Fathers, the weightiest argument for their teaching on papal infallibility was the witness provided by ecumenical councils. As proofs for the Church’s long-standing teaching on papal infallibility, Pastor aeternus selected statements from three councils: Constantinople IV (869-870), Lyons II (1274), and Florence (1439). Although the Orthodox no longer recognized them as truly ecumenical, churches from the East had participated in all three councils. For the drafters of Pastor aeternus, each one attests to the pope’s infallibility teaching authority.
As requested by the Roman delegates, the Fathers at Constantinople IV all signed the profession of faith drawn up by Pope Hormisdas (514-523). The Pope had sent the text to Constantinople in 515 in order to end the Acacian schism. This statement affirmed that “in the apostolic See the Catholic religion has been kept unsullied, and its teaching kept holy.” The First Vatican Council also invoked the confession of faith professed at the Second Council of Lyons, which attributed a primacy in teaching to the Roman Church. Disputes about faith, Lyons II aserted, had to be judged by Rome: “And, as she is bound above all to defend the truth of faith, they must be decided by her judgment.” The Fathers at Vatican I interpreted this right of the pope to resolve disputes in such a way that it involved the irrevocability of his judgment. Lastly, they invoked evidence from the Council of Florence. Its statement that “the Roman Pontiff is the true vicar of Christ, the head of the whole Church, father and teacher of all Christians” implicitly defined, they believed, the doctrine of papal infallibility. According to Vatican I, therefore, the teaching of these three ecumenical councils warrented the council’s conviction that Rome has assumed and been granted a supreme magisterium in the Church.