21 Can the power of baptism be greater or of more avail than confession, than suffering, when one confesses Christ before men and is baptized in his own blood? And yet even this baptism does not benefit a heretic, although he has confessed Christ, and been put to death outside the Church, unless the patrons and advocates of heretics declare that the heretics who are slain in a false confession of Christ are martyrs, and assign to them the glory and the crown of martyrdom contrary to the testimony of the apostle, who says that it will profit them nothing although they were burnt and slain. But if not even the baptism of a public confession and blood can profit a heretic to salvation, because there is no salvation out of the Church, how much less shall it be of advantage to him, if in a hiding-place and a cave of robbers, stained with the contagion of adulterous water, he has not only not put off his old sins, but rather heaped up still newer and greater ones!
Wherefore baptism cannot be common to us and to heretics,to whom neither God the Father, nor Christ the Son, nor the Holy Ghost, nor the faith, nor the Church itself, is common. And therefore it behooves those to be baptized who come from heresy to the Church, that so they who are prepared, in the lawful, and true, and only baptism of the holy Church, by divine regeneration, for the kingdom of God, may be born of both sacraments, because it is written, “Unless a man be born of water and of the Spirit, he cannot enter into the kingdom of God.”
Source: The Epistles of Cyprian (New Advent)