2 All the more, then, because "we are fighting for the honor and unity" of the Church, let us beware of giving to heretics the credit of whatever we acknowledged among them as belonging to the Church; but let us teach them by argument, that what they possess that is derived from unity is of no efficacy to their salvation, unless they shall return to that same unity. For "the water of the Church is full of faith, and salvation, and holiness" to those who use it rightly. No one, however, can use it well outside the Church. But to those who use it perversely, whether within or without the Church, it is employed to work punishment, and does not conduce to their reward. And so baptism "cannot be corrupted and polluted," though it be handled by the corrupt or by adulterers, just as also "the Church herself is uncorrupt, and pure, and chaste." And so no share in it belongs to the avaricious, or thieves, or usurers—many of whom, by the testimony of Cyprian himself in many places of his letters, exist not only without, but actually within the Church—and yet they both are baptized and do baptize, with no change in their hearts.
3. For this, too, he says, in one of his epistles to the clergy on the subject of prayer toGod, in which, after the fashion of the holy Daniel, he represents the sins of his people as falling upon himself. For among many other evils of which he makes mention, he speaks of them also as "renouncing the world in words only and not in deeds;" as the apostle says of certain men, "They profess that they know God, but in works they deny Him." These, therefore, the blessed Cyprian shows to be contained within the Church herself, who are baptized without their hearts being changed for the better, seeing that they renounce the world in words and not in deeds, as the Apostle Peter says, "The like figure whereunto even baptism does also now save us, (not the putting away of the filth of the flesh, but the answer of a good conscience)," which certainly they had not of whom it is said that they "renounced the world in words only, and not in deeds;" and yet he does his utmost, by chiding and convincing them, to make them at length walk in the way of Christ, and be His friends rather than friends of the world.
Source: On Baptism, Against the Donatists (New Advent)