29 With regard to the objection brought against Cyprian, that the catechumens who were seized in martyrdom, and slain for Christ's name's sake, received a crown even without baptism, I do not quite see what it has to do with the matter, unless, indeed, they urged that heretics could much more be admitted with baptism to Christ's kingdom, to which catechumens were admitted without it, since He Himself has said, "Unless a man be born of water and of the Spirit, he cannot enter into the kingdom of God." Now, in this matter I do not hesitate for a moment to place the Catholic catechumen, who is burning with love for God, before the baptized heretic; nor yet do we thereby do dishonor to the sacrament of baptism which the latter has already received, the former not as yet; nor do we consider that the sacrament of the catechumen is to be preferred to the sacrament of baptism, when we acknowledge that some catechumens are better and more faithful than some baptized persons.
For the centurion Cornelius, before baptism, was better than Simon, who had been baptized. For Cornelius, even before his baptism, was filled with the Holy Spirit; Simon, even after baptism, was puffed up with an unclean spirit. Cornelius, however, would have been convicted of contempt for so holy a sacrament, if, even after he had received the Holy Ghost, he had refused to be baptized. But when he was baptized, he received in no wise a better sacrament than Simon; but the different merits of the men were made manifest under the equal holiness of the same sacrament— so true is it that the good or ill deserving of the recipient does not increase or diminish the holiness of baptism.
But as baptism is wanting to a good catechumen to his receiving the kingdom of heaven, so true conversion is wanting to a bad man though baptized. For He who said, "Unless a man be born of water and of the Spirit, he cannot enter into the kingdom of God," said also Himself, "unless your righteousness shall exceed the righteousness of the scribes and Pharisees, you shall in no case enter into the kingdom of heaven." For that the righteousness of the catechumens might not feel secure, it is written, "Unless a man be born again of water and of the Spirit, he cannot enter into the kingdom of God."
And again, that the unrighteousness of the baptized might not feel secure because they had received baptism, it is written, "Unless your righteousness shall exceed the righteousness of the scribes and Pharisees, you shall in no case enter into the kingdom of heaven." The one were too little without the other; the two make perfect the heir of that inheritance. As, then, we ought not to depreciate a man's righteousness, which begins to exist before he is joined to the Church, as the righteousness of Cornelius began to exist before he was in the body of Christian men,— which righteousness was not thought worthless, or the angel would not have said to him, "Your prayers and your alms have come up as a memorial before God;" nor did it yet suffice for his obtaining the kingdom of heaven, or he would not have been told to send to Peter, — so neither ought we to depreciate the sacrament of baptism, even though it has been received outside the Church.
But since it is of no avail for salvation unless he who has baptism indeed in full perfection be incorporated into the Church, correcting also his own depravity, let us therefore correct the error of the heretics, that we may recognize what in them is not their own but Christ's.
Source: On Baptism, Against the Donatists (New Advent)