39 Hence, therefore, we have now set before us an easier and more simple consideration of that ark of which Noah was the builder and pilot. For Peter says that in the ark of Noah, "few, that is, eight souls, were saved by water. 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 towards God)." Wherefore, if those appear to men to be baptized in Catholic unity who renounce the world in words only and not in deeds, how do they belong to the mystery of this ark in whom there is not the answer of a good conscience?
Or how are they saved by water, who, making a bad use of holy baptism, though they seem to be within, yet persevere to the end of their days in a wicked and abandoned course of life? Or how can they fail to be saved by water, of whom Cyprian himself records that they were in time past simply admitted to the Church with the baptism which they had received in heresy? For the same unity of the ark saved them, in which no one has been saved except by water. For Cyprian himself says, "The Lord is able of His mercy to grant pardon, and not to sever from the gifts of His Church those who, being in all simplicity admitted to the Church, have fallen asleep within her pale." If not by water, how in the ark?
If not in the ark, how in the Church? But if in the Church, certainly in the ark; and if in the ark, certainly by water. It is therefore possible that some who have been baptized without may be considered, through the foreknowledge of God, to have been really baptized within, because within the water begins to be profitable to them unto salvation; nor can they be said to have been otherwise saved in the ark except by water. And again, some who seemed to have been baptized within may be considered, through the same foreknowledge of God, more truly to have been baptized without, since, by making a bad use of baptism, they die by water, which then happened to no one who was not outside the ark.
Certainly it is clear that, when we speak of within and without in relation to the Church, it is the position of the heart that we must consider, not that of the body, since all who are within in heart are saved in the unity of the ark through the same water, through which all who are in heart without, whether they are also in body without or not, die as enemies of unity. As therefore it was not another but the same water that saved those who were placed within the ark, and destroyed those who were left without the ark, so it is not by different baptisms, but by the same, that good Catholics are saved, and bad Catholics or heretics perish.
But what the most blessed Cyprian thinks of the Catholic Church, and how the heretics are utterly crushed by his authority; notwithstanding the much I have already said, I have yet determined to set forth by itself, if God will, with somewhat greater fullness and perspicuity, so soon as I shall have first said about his Council what I think is due from me, which, in God's will, I shall attempt in the following book.
Source: On Baptism, Against the Donatists (New Advent)