13 Therefore Cyprian writes to Jubaianus as follows, "concerning the baptism of heretics, who, being placed without, and set down out of the Church," seem to him to "claim to themselves a matter over which they have neither right nor power. Which we," he says, "cannot account valid or lawful, since it is clear that among them it is unlawful." Neither, indeed, do we deny that a man who is baptized among heretics, or in any schism outside the Church, derives no profit from it so far as he is partner in the perverseness of the heretics and schismatics; nor do we hold that those who baptize, although they confer the real true sacrament of baptism, are yet acting rightly, in gathering adherents outside the Church, and entertaining opinions contrary to the Church. But it is one thing to be without a sacrament, another thing to be in possession of it wrongly, and to usurp it unlawfully. Therefore they do not cease to be sacraments of Christ and the Church, merely because they are unlawfully used, not only by heretics, but by all kinds of wicked and impious persons. These, indeed, ought to be corrected and punished, but the sacraments should be acknowledged and revered.
14. Cyprian, indeed, says that on this subject not one, but two or more Councils were held; always, however, in Africa. For indeed in one he mentions that seventy-one bishops had been assembled, — to all whose authority we do not hesitate, with all due deference to Cyprian, to prefer the authority, supported by many more bishops, of the whole Church spread throughout the whole world, of which Cyprian himself rejoiced that he was an inseparable member.
15. Nor is the water "profane and adulterous" over which the name of God is invoked, even though it be invoked by profane and adulterous persons; because neither the creature itself of water, nor the name invoked, is adulterous. But the baptism of Christ, consecrated by the words of the gospel, is necessarily holy, however polluted and unclean its ministers may be; because its inherent sanctity cannot be polluted, and the divine excellence abides in its sacrament, whether to the salvation of those who use it aright, or to the destruction of those who use it wrong. Would you indeed maintain that, while the light of the sun or of a candle, diffused through unclean places, contracts no foulness in itself therefrom, yet the baptism of Christ can be defiled by the sins of any man, whatsoever he may be? For if we turn our thoughts to the visible materials themselves, which are to us the medium of the sacraments, every one must know that they admit of corruption. But if we think on that which they convey to us, who can fail to see that it is incorruptible, however much the men through whose ministry it is conveyed are either being rewarded or punished for the character of their lives?
Source: On Baptism, Against the Donatists (New Advent)