22 It is to no purpose, then, that they say to us, "If you acknowledge our baptism, what do we lack that should make you suppose that we ought to think seriously of joining your communion?" For we reply, We do not acknowledge any baptism of yours; for it is not the baptism of schismatics or heretics, but of God and of the Church, wheresoever it may be found, and wherever it may be transferred. But it is in no sense yours, except because you entertain false opinions, and do sacrilegious acts, and have impiously separated yourselves from the Church.
For if everything else in your practice and opinions were true, and still you were to persist in this same separation, contrary to the bond of brotherly peace, contrary to the union of all the brethren, who have been manifest, according to the promise, in all the world; the particulars of whose history, and the secrets of whose hearts, you never could have known or considered in every case, so as to have a right to condemn them; who, moreover, cannot be liable to condemnation for submitting themselves to the judges of the Church rather than to one of the parties to the dispute—in this one thing, at least, in such a case, you are deficient, in which he is deficient who lacks charity.
Why should we go over our argument again? Look and see yourselves in the apostle, how much there is that you lack. For what does it matter to him who lacks charity, whether he be carried away outside the Church at once by some blast of temptation, or remain within the Lord's harvest, so as to be separated only at the final winnowing? And yet even such, if they have once been born in baptism, need not be born again.
Source: On Baptism, Against the Donatists (New Advent)