42 Fortunatus of Thuccabori said: "Jesus Christ our Lord and God, the Son of God the Father and Creator, built His Church upon a rock, not upon heresy, and gave the power of baptizing to bishops, not to heretics. Wherefore those who are outside the Church, and stand against Christ, scattering His sheep and flock, cannot baptize outside."
43. He added the word "outside" in order that he might not be answered with a like brevity to Successus. For otherwise he might also have been answered word for word: Jesus Christ our Lord and God, the Son of God the Father and Creator, built His Church upon a rock, not upon iniquity, and gave the power of baptizing to bishops, not to the unrighteous. Wherefore those who do not belong to the rock on which they build, who hear the word of God and do it, but, living contrary to Christ in hearing the word and not doing it, and hereby building on the sand, in this way scatter His sheep and flock by the example of an abandoned character, cannot baptize. Might not this be said with all the semblance of truth? And yet it is false. For the unrighteous do baptize, since those robbers are unrighteous whom Cyprian maintained to be at unity with himself. But for this reason, says the Donatist, he adds "outside." Why therefore can they not baptize outside? Is it because they are worse from the very fact that they are outside? But it makes no difference, in respect of the validity of baptism, how much worse the minister may be. For there is not so much difference between bad and worse as between good and bad; and yet, when the bad baptizes, he gives the selfsame sacrament as the good. Therefore, also, when the worse baptizes, he gives the selfsame sacrament as the less bad. Or is it that it is not in respect of man's merit, but of the sacrament of baptism itself, that it cannot be given outside? If this were so, neither could it be possessed outside, and it would be necessary that a man should be baptized again so often as he left the Church and again returned to it.
44. Further, if we inquire more carefully what is meant by "outside," especially as he himself makes mention of the rock on which the Church is built, are not they in the Church who are on the rock, and they who are not on the rock, not in the Church either? Now, therefore, let us see whether they build their house upon a rock who hear the words of Christ and do them not. The Lord Himself declares the contrary, saying, "Whosoever hears these sayings of mine, and does them, I will liken him unto a wise man, which built his house upon a rock;" and a little later, "Every one that hears these sayings of mine, and does them not, shall be likened unto a foolish man, which built his house upon the sand." If, therefore, the Church is on a rock, those who are on the sand, because they are outside the rock, are necessarily outside the Church. Let us recollect, therefore, how many Cyprian mentions as placed within who build upon the sand, that is, who hear the words of Christ and do them not. And therefore, because they are on the sand, they are proved to be outside the rock, that is, outside the Church; yet even while they are so situated, and are either not yet or never changed for the better, not only do they baptize and are baptized, but the baptism which they have remains valid in them though they are destined to damnation.
45. Neither can it be said in this place, Yet who is there that does all the words of the Lord which are written in the evangelic sermon itself, at the end of which He says, that he who heard the said words and did them built upon a rock, and he who heard them and did them not built upon the sand? For, granting that by certain persons all the words are not accomplished, yet in the same sermon He has appointed the remedy, saying, "Forgive, and you shall be forgiven." And after the Lord's prayer had been recorded in detail in the same sermon, He says, "For I say unto you, if you forgive men their trespasses, your heavenly Father will also forgive you: but if you forgive not men their trespasses, neither will your Father forgive your trespasses." Hence also Peter says, "For charity shall cover the multitude of sins;" which charity they certainly did not have, and on this account they built upon the sand, of whom the same Cyprian says, that within the Church they held conversation, even in the time of the apostles, in unkindly hatred alien from Christian charity; and therefore they seemed indeed to be within, but really were without, because they were not on that rock by which the Church is signified.
Source: On Baptism, Against the Donatists (New Advent)