15 To the declaration of Polycarp of Adrumetum, that "those who declare the baptism of heretics to be valid, make ours of none effect," we answer, if that is the baptism of heretics which is given by heretics, then that is the baptism of the covetous and murderers which is given by them within the Church. But if this be not their baptism, neither is the other the baptism of heretics; and so it is Christ's, by whomsoever it be given.
Source: On Baptism, Against the Donatists (New Advent)