98 It is indeed worth while to consider the whole of the passage in the aforesaid letter to Magnus, which he has put together as follows: "Not dwelling," he says, "in the house of God— that is to say, in the Church of Christ— in which none dwell save those that are of one heart and of one mind, as the Holy Spirit says in the Psalms, speaking of 'God that makes men to be of one mind in an house.' Finally, the very sacrifices of the Lord declare that Christians are united among themselves by a firm and inseparable love for one another.
For when the Lord calls bread, which is compacted together by the union of many grains, His body, He is signifying one people, whom He bore, compacted into one body; and when He calls wine, which is pressed out from a multitude of branches and clusters and brought together into one, His blood, He also signifies one flock joined together by the mingling of a multitude united into one." These words of the blessed Cyprian show that he both understood and loved the glory of the house of God, which house he asserted to consist of those who are of one heart and of one mind, proving it by the testimony of the prophets and the meaning of the sacraments, and in which house certainly were not found those envious persons, those malevolent without charity, who nevertheless used to baptize.
From whence it is clear that the sacrament of Christ can both be in and be administered by those who are not in the Church of Christ, in which Cyprian himself bears witness that there are none dwelling save those who are of one heart and of one mind. Nor can it indeed be said that they are allowed to baptize so long as they are undetected, seeing that the Apostle Paul did not fail to detect those of whose ministry he bears unquestionable testimony in his epistle, saying that he rejoices that they also were proclaiming Christ. For he says of them, "Whether in pretense or in truth, Christ is preached; and I therein do rejoice, yea, and will rejoice."
Source: On Baptism, Against the Donatists (New Advent)