You should understand that the head of the Apostles was Saint Peter, to whom Christ said, “You are the rock; and on this rock I shall build my church, and the gates of hell will not overcome it.” After his resurrection, he also said to him three times, while on the shore of the sea of Tiberius, “Simon, do you love me? Feed my lambs, rams and ewes.” In another passage, he said to him, “Simon, Satan will ask to sift you like wheat, and I prayed that you not lose your faith; but you, at that time, have compassion on your brethren and strengthen them.” Do you not see that Saint Peter is the foundation of the Church, selected to shepherd it, that those who believe in his faith will never lose their faith, and that he was ordered to have compassion on his brethren and to strengthen them?
As for Christ’s words, “I have prayed for you, that you not lose your faith; but you, have compassion on your brethren, at that time, and strengthen them”, we do not think that he meant Saint Peter himself. Rather, he meant nothing more than the holders of the seat of Saint Peter, that is, Rome. Just as when he said to the apostles, “I am with you always, until the end of the age”, he did not mean just the apostles themselves, but also those who would be in charge of their seats and their flocks; in the same way, when he spoke his last words to Saint Peter, “Have compassion, at that time, and strengthen your brethren; and your faith will not be lost”, he meant by this nothing other than the holders of his seat.
Yet another indication of this is the fact that among the Apostles it was Saint Peter alone who lost his faith and denied Christ, which Christ may have allowed to happen to Peter so as to teach us that it was not Peter that he meant by these words. Moreover, we know of no Apostle who fell and needed Saint Peter to strengthen him. If someone says that Christ meant by these words only Saint Peter himself, this person causes the Church to lack someone to strengthen it after the death of Saint Peter. How could this happen, especially when we see all the sifting of the Church that came from Satan after the Apostles’ death? All of this indicates that Christ did not mean them by these words. Indeed, everyone knows that the heretics attacked the Church only after the death of the Apostles – Paul of Samosata, Arius, Macedonius, Eunomius, Sabelllius, Apollinaris, Origen, and others. If he meant by these words in the Gospel only Saint Peter, the Church would have been deprived of comfort and would have had no one to deliver her from those heretics, whose heresies are truly “the gates of hell”, which Christ said would not overcome the Church. Accordingly, there is no doubt that he meant by these words nothing other than the holders of the seat of Saint Peter, who have continually strengthened their brethren and will not cease to do so as long as this present age lasts.
– From On the Councils by Theodore Abu Qurrah, Bishop of Haran, Syria (+820)
As for us, through the grace of the Holy Spirit, our sole goal is to build ourselves on the foundation of Saint Peter, he who directed the six holy councils. These councils were gathered by command of the Bishop of Rome, the city of the world. Whoever sits on that city’s throne is authorized by Christ to have compassion on the people of the Church, by summoning the ecumenical council, and to strengthen them, even as we have demonstrated in other places. We ask Christ to confirm us in this forever, that we might inherit through it his kingdom, in that we have joined with it the doing of his commandments. To him be praise, along with the Father and the Holy Spirit, forever and forever.
– From On the Death of Christ by the same author