10 Iambus of Germaniciana said: "Those who approve the baptism of heretics disapprove ours, so as to deny that such as are, I will not say washed, but defiled outside the Church, ought to be baptized within the Church."
11. To him we answer, that none of our party approves the baptism of heretics, but all the baptism of Christ, even though it be found in heretics who are as it were chaff outside the Church, as it may be found in other unrighteous men who are as chaff within the Church. For if those who are baptized without the Church are not washed, but defiled, assuredly those who are baptized outside the rock on which the Church is built are not washed, but defiled. But all are without the said rock who hear the words of Christ and do them not. Or if it be the case that they are washed indeed in baptism, but yet continue in the defilement of their unrighteousness, from which they were unwilling to be changed for the better, the same is true also of the heretics.
Source: On Baptism, Against the Donatists (New Advent)