23 What, then, is to be made of what is written, “Abstain from strange water, and drink not from a strange fountain,” if, leaving the sealed fountain of the Church, you take up strange water for your own, and pollute the Church with unhallowed fountains? For when you communicate with the baptism of heretics, what else do you do than drink from their slough and mud; and while you yourself are purged with the Church's sanctification, you become befouled with the contact of the filth of others?
And do you not fear the judgment of God when you are giving testimony to heretics in opposition to the Church, although it is written, “A false witness shall not be unpunished?” But indeed you are worse than all heretics. For when many, as soon as their error is known, come over to you from them that they may receive the true light of the Church, you assist the errors of those who come, and, obscuring the light of ecclesiastical truth, you heap up the darkness of the heretical night; and although they confess that they are in sins, and have no grace, and therefore come to the Church, you take away from them remission of sins, which is given in baptism, by saying that they are already baptized and have obtained the grace of the Church outside the Church, and you do not perceive that their souls will be required at your hands when the day of judgment shall come, for having denied to the thirsting the drink of the Church, and having been the occasion of death to those that were desirious of living.
And, after all this, you are indignant!
Source: The Epistles of Cyprian (New Advent)