32 Marcellus of Zama said: "Since sins are remitted only in the baptism of the Church, he who does not baptize a heretic holds communion with a sinner."
33. What, does he who holds communion with one who does this not hold communion with a sinner? But what else did all of them do, "in judging no one, or removing from the right of communion any one who entertained a different opinion"? Where, then, is the Church? Are those things not an obstacle to those who are patient, and tolerate the tares lest the wheat should be rooted out together with them? I would have them therefore say, who have committed the sacrilege of schism by separating themselves from the whole world, how it comes that they have in their mouths the judgment of Cyprian, while they do not have in their hearts the patience of Cyprian. But to this Marcellus we have an answer in what has been said above concerning baptism and the remission of sins, explaining how there can be baptism in a man although there be in him no remission of his sins.
Source: On Baptism, Against the Donatists (New Advent)