“The Lord says to Peter: ‘I say to you,’ he says, ‘that you are Peter, and upon this rock I will build my Church, and the gates of hell will not overcome it. And to you I will give the keys of the kingdom of heaven . . . ’ [Matt. 16:18–19]. On him [Peter] he builds the Church, and to him he gives the command to feed the sheep [John 21:17], and although he assigns a like power to all the apostles, yet he founded a single chair [cathedra], and he established by his own authority a source and an intrinsic reason for that unity. Indeed, the others were that also which Peter was [i.e., apostles], but a primacy is given to Peter, whereby it is made clear that there is but one Church and one chair. . . . If someone does not hold fast to this unity of Peter, can he imagine that he still holds the faith? If he [should] desert the chair of Peter upon whom the Church was built, can he still be confident that he is in the Church?” (The Unity of the Catholic Church 4; 1st edition [A.D. 251]).
“There is one God and one Christ, and one Church, and one chair founded on Peter by the word of the Lord. It is not possible to set up another altar or for there to be another priesthood besides that one altar and that one priesthood. Whoever has gathered elsewhere is scattering” (Letters 43:5 [A.D. 253]).
“There [John 6:68–69] speaks Peter, upon whom the Church would be built, teaching in the name of the Church and showing that even if a stubborn and proud multitude withdraws because it does not wish to obey, yet the Church does not withdraw from Christ. The people joined to the priest and the flock clinging to their shepherd are the Church. You ought to know, then, that the bishop is in the Church and the Church in the bishop, and if someone is not with the bishop, he is not in the Church. They vainly flatter themselves who creep up, not having peace with the priests of God, believing that they are
secretly [i.e., invisibly] in communion with certain individuals. For the Church, which is one and Catholic, is not split nor divided, but it is indeed united and joined by the cement of priests who adhere one to another” (ibid., 66:8).