46 Sedatus of Tuburbo said: "Inasmuch as water, sanctified by the prayer of the priest in the Church, washes away sins, just so much does it multiply sins when infected, as by a cancer, with the words of heretics. Wherefore one must strive, with all such efforts as conduce to peace, that no one who has been infected and tainted by heretical error should refuse to receive the one true baptism, with which whosoever is not baptized shall not inherit the kingdom of heaven."
47. To this we answer, that if the water is not sanctified, when through want of skill the priest who prays utters some words of error, many, not only of the bad, but of the good brethren in the Church itself, fail to sanctify the water. For the prayers of many are corrected every day on being recited to men of greater learning, and many things are found in them contrary to the Catholic faith. Supposing, then, that it were shown that some persons were baptized when these prayers had been uttered over the water, will they be bidden to be baptized afresh? Why not? Because generally the fault in the prayer is more than counterbalanced by the intent of him who offers it; and those fixed words of the gospel, without which baptism cannot be consecrated, are of such efficacy, that, by their virtue, anything faulty that is uttered in the prayer contrary to the rule of faith is made of no effect, just as the devil is excluded by the name of Christ. For it is clear that if a heretic utters a faulty prayer, he has no good intent of love whereby that want of skill may be compensated, and therefore he is like any envious or spiteful person in the Catholic Church itself, such as Cyprian proves to exist within the Church. Or one might offer some prayer, as not unfrequently happens, in which he should speak against the rule of faith, since many rush into the use of prayers which are composed not only by unskillful men who love to talk, but even by heretics, and in the simplicity of ignorance, not being able to discern their true character, use them, thinking they are good; and yet what is erroneous in them does not vitiate what is right, but rather it is rendered null thereby, just as in the man of good hope and approved faith, who yet is but a man, if in anything he be otherwise minded, what he holds aright is not thereby vitiated until God reveal to him also that in which he is otherwise minded. But supposing that the man himself is wicked and perverse, then, if he should offer an upright prayer, in no part contrary to the Catholic faith, it does not follow that because the prayer is right the man himself is also right; and if over some he offer an erroneous prayer, God is present to uphold the words of His gospel, without which the baptism of Christ cannot be consecrated, and He Himself consecrates His sacrament, that in the recipient, either before he is baptized, or when he is baptized, or at some future time when he turns in truth to God, that very sacrament may be profitable to salvation, which, were he not to be converted, would be powerful to his destruction. But who is there who does not know that there is no baptism of Christ, if the words of the gospel in which consists the outward visible sign be not forthcoming? But you will more easily find heretics who do not baptize at all, than any who baptize without those words. And therefore we say, not that every baptism (for in many of the blasphemous rites of idols men are said to be baptized), but that the baptism of Christ, that is, every baptism consecrated in the words of the gospel, is everywhere the same, and cannot be vitiated by any perversity on the part of any men.
48. We must certainly not lightly pass over in this judgment that he here inserted a clause, and says, "Wherefore we must strive, with all such efforts as conduce to peace, that no one who has been infected," etc. For he had regard to those words of the blessed Cyprian in his opening speech, "Judging no man, nor depriving any of the right of communion if he entertain a different view." See of what power is the love of unity and peace in the good sons of the Church, that they should choose rather to show tolerance towards those whom they called sacrilegious and profane, being admitted, as they thought, without the sacrament of baptism, if they could not correct them as they thought was right, than on their account to break that holy bond, lest on account of the tares the wheat also should be rooted out, — permitting, so far as rested with them, as in that noblest judgment of Solomon, that the infant body should rather be nourished by the false mother than be cut in pieces. But this was the opinion both of those who held the truer view about the sacrament of baptism, and of those to whom God, in consideration of their great love, was purposing to reveal any point in which they were otherwise minded.
Source: On Baptism, Against the Donatists (New Advent)