St. Ambrose turns against the Novatians themselves another objection concerning blasphemy against the Holy Spirit, showing that it consists in an erroneous belief, proving this by St. Peter's words against Simon Magus, and other passages, exhorting the Novatians to return to the Church, affirming that such is our Lord's mercy that even Judas would have found forgiveness had he repented.
20. But we have heard that you are accustomed to bring forward as an objection that which is written: “Every sin and blasphemy shall be forgiven unto men, but blasphemies against the Spirit shall not be forgiven unto men. And whosoever shall speak a word against the Son of Man, it shall be forgiven him, but whosoever shall speak against the Holy Spirit, it shall not be forgiven him, neither in this world, nor in that which is to come.” By which quotation the whole of your assertion is destroyed and done away, for it is written: “Every sin and blasphemy shall be forgiven unto men.” Why, then, do you not remit them? Why do you bind chains which you do not loose? Why do you tie knots which you do not unfasten? Forgive the others, and deal with those who you think are bound for ever by the authority of the Gospel for sinning against the Holy Spirit.
21. But let us consider the case of those whom the Lord so binds, going back to the words before the passage quoted, that we may understand it more clearly: The Jews were saying: “This man does not cast out devils, but by Beelzebub, prince of the devils.” Jesus replied: “Every kingdom divided against itself shall be destroyed, and every city or house divided against itself shall not stand; for if Satan casts out Satan, he is divided against himself, how then shall his kingdom stand? But if I cast out devils by Beelzebub, by whom do your sons cast them out?”
22. Now we see plainly here that the words are expressly used of those who were saying that the Lord Jesus cast out devils through Beelzebub, to whom the Lord gave that answer, because they were of the heritage of Satan, who compared the Saviour of all to Satan, and attributed the grace of Christ to the kingdom of the devil. And that we might know that He was speaking of this blasphemy, He added: “O generation of vipers, how can you speak good, being yourselves evil?” He says, then, that those who thus speak attain not to forgiveness.
23. Then, when Simon, depraved by long practice of magic, had thought he could gain by money the power of conferring the grace of Christ and the infusion of the Holy Spirit, Peter said: “You have neither part nor lot in this faith, for your heart is not right with God. Repent therefore of this your wickedness, and pray the Lord, if perchance this thought of your heart may be forgiven you, for I see that you are in the bond of iniquity and in the bitterness of gall.” We see that Peter by his apostolic authority condemns him who blasphemes against the Holy Spirit through magic vanity, and all the more because he had not the clear consciousness of faith. And yet he did not exclude him from the hope of forgiveness, for he called him to repentance.
24. The Lord then replies to the blasphemy of the Pharisees, and refuses to them the grace of His power, which consists in the remission of sins, because they asserted that His heavenly power rested on the help of the devil. And He affirms that they act with satanic spirit who divide the Church of God, so that He includes the heretics and schismatics of all times, to whom He denies forgiveness, for every other sin is concerned with single persons, this is a sin against all. For they alone wish to destroy the grace of Christ who rend asunder the members of the Church for which the Lord Jesus suffered, and the Holy Spirit was given us.
25. Lastly, that we may know that He is speaking of those who destroy the unity of the Church, we find it written: “He that is not with Me is against Me, and he that gathered not with Me, scatters.” And that we might know that He is speaking of these, He at once added: “Therefore I say unto you, every sin and blasphemy shall be forgiven unto men, but blasphemies against the Spirit shall not be forgiven unto men.” When He says, “Therefore say I unto you,” is it not evident that He intended the words following to be laid to heart by us beyond the others? And He rightly added: “A good tree brings forth good fruits, but a bad tree brings forth bad fruits,” for an evil association cannot produce good fruits. The tree, then, is the association; the fruits of the good tree are the children of the Church.
26. Return, then, to the Church, those of you who have wickedly separated yourselves. For He promises forgiveness to all who are converted, since it is written: “Whosoever shall call on the Name of the Lord shall be saved.” And lastly, the Jewish people who said of the Lord Jesus, “He has a devil,” and “He casts out devils through Beelzebub,” and who crucified the Lord Jesus, are, by the preaching of Peter, called to baptism, that they may put away the guilt of so great a wickedness.
27. But what wonder is it if you should deny salvation to others, who reject your own, though they lose nothing who seek for penance from you? For I suppose that even Judas might through the exceeding mercy of God not have been shut out from forgiveness, if he had expressed his sorrow not before the Jews but before Christ. “I have sinned,” he said, “in that I have betrayed righteous blood.” Their answer was: “What is that to us, you see to that.” What other reply do you give, when one guilty of a smaller sin confesses his deed to you? What do you answer but this: “What is that to us, you see to that”? The halter followed on those words, but the punishment is all the more severe, the smaller the sin is.
28. But if they be not converted, do you at least repent, who by many a slip have fallen from the lofty pinnacle of innocence and faith. We have a good Lord, Whose will it is to forgive all, Who called you by the prophet, and said: “I, even I, am He that blots out transgressions, and I will not remember, but you remember, and let us plead together.”
Source: On Repentance (New Advent)