Firmilian of Caesarea, Letter to Cyprian 21
Someone may ask, "What will become of those who were admitted without the Church's baptism after coming from heretics?" If they have already died, we count them among those who were catechized here but passed away before receiving baptism. They not only gained the benefit of the truth and faith they reached after abandoning their error, even though death prevented them from receiving the full completion of grace. As for those who are still alive, they must be baptized with the Church's baptism so that they may obtain the forgiveness of sins. Otherwise, relying on someone else's presumption, they would remain in their former error and die without the completion of grace. Besides, what fault lies either with those who admit them or with those who are admitted, that without having their stains washed away in the Church's bath and without having their sins laid bare, they dare to touch the body and blood of the Lord by rashly claiming a share in the communion? For it is written: "Whoever eats the bread or drinks the cup of the Lord unworthily will be guilty of the body and blood of the Lord" (1 Corinthians 11:27).
Latin
Quid ergo, inquiunt, fiet de his qui ab haereticis venientes sine Ecclesiae baptismo admissi sunt? Si de saeculo excesserunt, in eorum numero habentur qui apud nos catechisati quidem sunt, sed prius quam baptizarentur obierunt, non modo dicunt emolumentum veritatis et fidei, ad quam, relicto errore, pervenerant, etsi consummationem gratiae consecuti non sunt, morte praeventi. Qui autem adhuc in saeculo permanent baptizentur Ecclesiae Baptismo, ut remissionem peccatorum consequi possint, ne, per alienam praesumptionem, in pristino errore manentes, sine gratiae consummatione moriantur. Caeterum quale delictum est, vel illorum qui admittunt, vel eorum qui admittuntur, ut non ablutis per Ecclesiae lavacrum sordibus, nec peccatis expositis, usurpata temere communicatione contingant corpus et sanguinem Domini, cum scriptum sit: Quicumque ederit panem aut biberit calicem Domini indigne, reus erit corporis et sanguinis Domini.