Papal Infallibility in the First Seven Ecumenical Councils

The Third Ecumenical Council

Session 1 - “[If] your holiness [i.e. the Patriarch Nestorius] have not a mind to [accept] the limits defined in the writings of [Pope] Celestine, Bishop of the Church of Rome, be well assured then that you have no lot with us, nor place or standing among the priests and bishops of God.” source

Session 2 - “The Apostolic and holy see of the most holy bishop Celestine, has previously given a decision and type in this matter, through the writings which were sent to the most God-beloved bishops... This we have also followed and...we carried into effect the type, having pronounced against him a canonical and judgment.” source

The Fourth Ecumenical Council

Session 2 - [After a declaration of the faith by Pope St. Leo the Great.] “This is the faith of the fathers, this is the faith of the Apostles. So we all believe, thus the orthodox believe. Anathema to him who does not thus believe! Peter has spoken thus through Leo! So taught the Apostles. Piously and truly did Leo teach, so taught [St.] Cyril [of Alexandria].” source

This may be the first instance where an ecumenical council recognizes that St. Peter speaks through his successors at least under the right conditions. But St. Peter, being in heaven, cannot err. Therefore, at least in this case, the pope spoke infallibly, and this supports papal infallibility by giving an instance of it.

Letter to Pope Leo I - “[Pope Leo is] the mouthpiece unto all of the blessed Peter...imparting the blessedness of his Faith unto all.” source

The Sixth Ecumenical Council

Session 4 - “[The Roman see] 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 Saviour himself.” (Citing Luke 22:31-32) source

And: “[Peter] received from the Redeemer of all himself, by three commendations, the duty of feeding the spiritual sheep of the Church; under [his] protecting shield, [the Roman Church] has never turned away from the path of truth in any direction of error, whose authority, as that of the Prince of all the Apostles, the whole Catholic Church, and the Ecumenical Synods have faithfully embraced, and followed in all things.” (ibid.) source

Letter to Pope Agatho - “Christ our true God...gave [us] a wise physician, namely your [self], to drive [heresies] away...by the remedies of orthodoxy, and to give the strength of health to the members of the church. Therefore to you, as to the bishop of the first see of the Universal Church, we leave what must be done, since you willingly take for your standing ground the firm rock of the faith, as we know from having read your [letter] to the most pious emperor: and we acknowledge that this letter was divinely written as by the Chief of the Apostles.” source

The Seventh Ecumenical Council

Session 2 - “[The letter] to the Romans commend[s] their zeal for the true faith… It is necessary to follow out this witness, and he that would contradict it is without good sense. Wherefore Hadrian, the ruler of Old Rome, since he was a sharer of these things, thus borne witness to, wrote [to the emperor]...confirming admirably and beautifully the ancient tradition of the Catholic Church… so [we] have confessed, so [we] do confess, and [we] so will confess… And the holy Synod said: The whole holy Synod thus teaches.” source

Session 2 - “[You will be] renowned and praised through the whole world...if you follow the tradition of the orthodox Faith of the Church of the holy Peter and Paul, the chief Apostles, and embrace their Vicar… For the holy and chief Apostles themselves, who set up the Catholic and orthodox Faith, have laid it down as a written law that all who after them are to be successors of their seats, should hold their Faith and remain in it to the end.” source