1 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 Scriptures suggest, 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.” 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 heathen man and a publican.” 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.
Source: The Epistles of Cyprian (New Advent)