Cyprian to Magnus his son, greeting. With your usual religious diligence, you have consulted my poor intelligence, dearest son, as to whether, among other heretics, they also who come from Novatian ought, after his profane washing, to be baptized, and sanctified in the Catholic Church, with the lawful, and true, and only baptism of the Church. Respecting which matter, as much as the capacity of my faith and the sanctity and truth of the divine Scripturessuggest, I answer, that no heretics and schismatics at all have any power or right. For which reason Novatian neither ought to be nor can be expected, inasmuch as he also is without the Church and acting in opposition to the peace and love of Christ, from being counted among adversaries and antichrists. For our Lord Jesus Christ, when He testified in His Gospel that those who were not with Him were His adversaries, did not point out any species of heresy, but showed that all whatsoever who were not with Him, and who, not gathering with Him, were scattering His flock, were His adversaries; saying, He that is not with me is against me, and he that gathers not with me scatters. Moreover, the blessed Apostle John himself distinguished no heresy or schism, neither did he set down any as specially separated; but he called all who had gone out from the Church, and who acted in opposition to the Church, antichrists, saying, You have heard that Antichrist comes, and even now have come many antichrists; wherefore we know that this is the last time. They went out from us, but they were not of us; for if they had been of us, they would have continued with us. 1 John 2:18-19 Whence it appears, that all are adversaries of the Lord and antichrists, who are known to have departed from charity and from the unity of the Catholic Church. In addition, moreover, the Lord establishes it in His Gospel, and says, But if he neglect to hear the Church, let him be unto you as a heathenman and a publican. Matthew 18:17 Now if they who despise the Church are counted heathens and publicans, much more certainly is it necessary that rebels and enemies, who forge false altars, and lawless priesthoods, and sacrilegious sacrifices, and corrupter names, should be counted among heathens and publicans; since they who sin less, and are only despisers of the Church, are by the Lord’s sentence judged to be heathens and publicans.
. . . For the faith of the sacred Scripture sets forth that the Church is not without, nor can be separated nor divided against itself, but maintains the unity of an inseparable and undivided house; since it is written of the sacrament of the passover, and of the lamb, which Lamb designated Christ: In one house shall it be eaten: you shall not carry forth the flesh abroad out of the house. Exodus 12:46 Which also we see expressed concerning Rahab, who herself also bore a type of the Church, who received the command which said, You shall bring your father, and your mother, and your brethren, and all your father’s household unto you into your house; and whosoever shall go out of the doors of your house into the street, his blood shall be upon him. Joshua 2:18-19 In which mystery is declared, that they who will live, and escape from the destruction of the world, must be gathered together into one house alone, that is, into the Church; but whosoever of those thus collected together shall go out abroad, that is, if any one, although he may have obtained grace in the Church, shall depart and go out of the Church, that his blood shall be upon him; that is, that he himself must charge it upon himself that he perishes; which the Apostle Paul explains, teaching and enjoining that a heretic must be avoided, as perverse, and a sinner, and as condemned of himself. For that man will be guilty of his own ruin, who, not being cast out by the bishop, but of his own accord deserting from the Church is by heretical presumption condemned of himself. —from Epistle 75  (c. 250)