Firmilian of Caesarea, Letter to Cyprian 8
Since Stephanus and his allies claim that forgiveness of sins and a second birth can take place in the baptism of heretics—even though they themselves admit that the Holy Spirit is not present among them—they ought to consider and understand that a spiritual birth cannot happen without the Holy Spirit. The blessed apostle Paul demonstrates this: before the Lord had sent the Holy Spirit, he baptized again with a spiritual baptism those who had received John’s baptism, and then laid hands on them so that they might receive the Holy Spirit (Acts 19:1-6). So when we see Paul rebaptizing his disciples after John’s baptism, why should we hesitate to baptize those who come to the Church from heresy after their illicit and profane dipping? Unless, of course, these bishops—to whom Paul was inferior—are able to give the Holy Spirit to incoming heretics through the laying on of hands alone, while Paul was not qualified to confer the Holy Spirit by the laying on of hands on those baptized by John unless he first baptized them with the baptism of the Church.
Latin
Et quoniam Stephanus et qui illi consentiunt contendunt dimissionem peccatorum et secundam nativitatem in haereticorum Baptisma posse procedere, apud quos etiam ipsi confitentur Spiritum sanctum non esse, considerent et intelligant spiritalem nativitatem sine Spiritu esse non posse, secundum quod et beatus apostolus Paulus eos qui ab Joanne baptizati fuerant, priusquam missus esset Spiritus sanctus a Domino, baptizavit denuo spiritali Baptismo, et sic eis manum imposuit ut acciperent Spiritum sanctum. Quale est autem ut cum Paulum post Joannis baptisma iterato discipulos suos baptizasse videamus, nec eos qui ab haeresi ad Ecclesiam veniunt post illicitam et profanam eorum tinctionem baptizare dubitemus? Nisi si his episcopis quibus [nunc] minor fuit Paulus, ut hi quidem possint per solam manus impositionem venientibus haereticis dare Spiritum sanctum, Paulus autem idoneus non fuerit qui ab Joanne baptizatis Spiritum sanctum per manus impositionem daret, nisi eos prius etiam Ecclesiae Baptismo baptizasset.