Saint Cyprian on Unity

"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: and whatever things you bind on earth shall be bound also in heaven, and whatever you loose on earth, they shall be loosed also in heaven.' And again He says to him after His resurrection: 'Feed my sheep.' On him He builds the Church, and to him He gives the command to feed the sheep; and although He assigns a like power to all the Apostles, yet He founded a single chair, 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; but a primacy is given to Peter whereby it is made clear that there is but one Church and one chair. So too, all are shepherds, and the flock is shown to be one, fed by all the Apostles in single-minded accord. If someone does not hold fast to this unity of Peter, can he imagine that he still holds the faith? If he desert the chair of Peter upon whom the Church was built, can he still be confident that he is in the Church? ...

"With a false bishop appointed for themselves by heretics, they dare even to set sail and carry letters from schismatics and blasphemers to the chair of Peter and to the principal Church, in which sacerdotal unity has its source; nor did they take thought that these are Romans, whose faith was praised by the preaching Apostle, and among whom it is not possible for perfidy to have entrance." ...

"You wrote also, that I should forward to Cornelius [bishop of Rome], our colleague, a copy of your letter, so that he might put aside any anxiety and know immediately that you are in communion with him, that is, with the Catholic Church.... Cornelius was made bishop [of Rome] by the decision of God and of His Christ, by the testimony of almost all the clergy, by the applause of the people then present, by the college of venerable priests and good men, . . . which is the place of Peter, the dignity of the sacerdotal chair.... Since it has been occupied both at the will of God and with the ratified consent of all of us, whoever wishes now to become bishop must do so outside. For he cannot have ecclesiastical rank who does not hold to the unity of the Church." ...

"There 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." ...

"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."

See also St. Cyprian's letter to Pope Stephen (254-257) requesting Pope Stephen to excommunicate Marcian, bishop of Arles (because of Marcian's Novatianism), and to oversee the appointment of a replacement for Marcian of Arles.