20 There is therefore "no fellowship between righteousness and unrighteousness," not only without, but also within the Church; for "the Lord knows them that are His," and "Let every one that names the name of Christ depart from iniquity." There is also "no communion between light and darkness," not only without, but also within the Church; for "he that hates his brother is still in darkness." And they at any rate hated Paul, who, preaching Christ of envy and malicious strife, supposed that they added affliction to his bonds; and yet the same Cyprian understands these still to have been within the Church. Since, therefore, "neither darkness can enlighten, nor unrighteousness justify," as Cyprian again says, I ask, how could those men baptize within the very Church herself? I ask, how could those vessels which the large house contains not to honor, but to dishonor, administer what is holy for the sanctifying of men within the great house itself, unless because that holiness of the sacrament cannot be polluted even by the unclean, either when it is given at their hands, or when it is received by those who in heart and life are not changed for the better? Of whom, as situated within the Church, Cyprian himself says, "Renouncing the world in word only, and not in deed."
21. There are therefore also within the Church "enemies of God, whose hearts the spirit of Antichrist has possessed;" and yet they, "deal with spiritual and divine things," which cannot profit for their salvation so long as they remain such as they are; and yet neither can they pollute them by their own uncleanness. With regard to what he says, therefore, "that they have no part given them in the saving grace of the Church, who, scattering and fighting against the Church of Christ, are called adversaries by Christ Himself, and antichrists by His apostles, this must be received under the consideration that there are men of this kind both within and without. But the separation of those that are within from the perfection and unity of the dove is not only known in the case of some men to God, but even in the case of some to their fellow-men; for, by regarding their openly abandoned life and confirmed wickedness, and comparing it with the rules of God's commandments, they understand to what a multitude of tares and chaff, situated now some within and some without, but destined to be most manifestly separated at the last day, the Lord will then say, "Depart from me, you that work iniquity," and "Depart into everlasting fire, prepared for the devil and his angels."
Source: On Baptism, Against the Donatists (New Advent)