20 Accordingly, if Marcion consecrated the sacrament of baptism with the words of the gospel, "In the name of the Father, and of the Son, and of the Holy Ghost," the sacrament was complete, although his faith expressed under the same words, seeing that he held opinions not taught by the Catholic truth, was not complete, but stained with the falsity of fables. For under these same words, "In the name of the Father, and of the Son, and of the Holy Ghost," not Marcion only, or Valentinus, or Arius, or Eunomius, but the carnal babes of the Church themselves (to whom the apostle said, "I could not speak unto you as unto spiritual, but as unto carnal"), if they could be individually asked for an accurate exposition of their opinions, would probably show a diversity of opinions as numerous as the persons who held them, "for the natural man receives not the things of the Spirit of God."
Can it, however, be said on this account that they do not receive the complete sacrament? Or that, if they shall advance, and correct the vanity of their carnal opinions, they must seek again what they had received? Each man receives after the fashion of his own faith; yet how much does he obtain under the guidance of that mercy of God, in the confident assurance of which the same apostle says, "If in anything ye be otherwise minded, God shall reveal even this unto you"? Yet the snares of heretics and schismatics prove for this reason only too pernicious to the carnally-minded, because their very progress is intercepted when their vain opinions are confirmed in opposition to the Catholic truth, and the perversity of their dissension is strengthened against the Catholic peace.
Yet if the sacraments are the same, they are everywhere complete, even when they are wrongly understood, and perverted to be instruments of discord, just as the very writings of the gospel, if they are only the same, are everywhere complete, even though quoted with a boundless variety of false opinions. For as to what Jeremiah says:— "Why do those who grieve me prevail against me? My wound is stubborn, whence shall I be healed? In its origin it became unto me as lying water, having no certainty," — if the term "water" were never used figuratively and in the allegorical language of prophecy except to signify baptism, we should have trouble in discovering what these words of Jeremiah meant; but as it is, when "waters" are expressly used in the Apocalypse to signify "peoples," I do not see why, by "lying water having no certainty," I should not understand, a "lying people, whom I cannot trust."
Source: On Baptism, Against the Donatists (New Advent)