5 Thus having defended Himself, and refuted their objection, and proved the vanity of their shameless dealings, He proceeds to alarm them. For this too is no small part of advice and correction, not only to plead and persuade, but to threaten also; which He does in many passages, when making laws and giving counsel.
And though the saying seem to have much obscurity, yet if we attend, its solution will prove easy.
First then it were well to listen to the very words: “All manner of sin and blasphemy shall be forgiven unto men; but the blasphemy of the Holy Ghost shall not be forgiven unto them. And whosoever speaks a word against the Son of Man, it shall be forgiven him; but whosoever speaks against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come.”
What now is it that He affirms? Many things have ye spoken against me; that I am a deceiver, an adversary of God. These things I forgive you on your repentance, and exact no penalty of you; but blasphemy against the Spirit shall not be forgiven, no, not to those who repent. And how can this be right? For even this was forgiven upon repentance. Many at least of those who said these words believed afterward, and all was forgiven them. What is it then that He says? That this sin is above all things unpardonable. Why so? Because Himself indeed they knew not, who He might be, but of the Spirit they received ample experience. For the prophets also by the Spirit said whatever they said; and indeed all in the Old Testament had a very high notion of Him.
What He says, then, is this: Be it so: you are offended at me, because of the flesh with which I am encompassed: can you say of the Spirit also, We know it not? And therefore is your blasphemy unpardonable, and both here and hereafter shall you suffer punishment. For many indeed have been punished here only (as he who had committed fornication, as they who partook unworthily of the mysteries, among the Corinthians); but you, both here and hereafter.
Now as to your blasphemies against me, before the cross, I forgive them: and the daring crime too of the cross itself; neither shall you be condemned for your unbelief alone. (For neither had they, that believed before the cross, perfect faith. And on many occasions He even charges them to make Him known to no man before the Passion; and on the cross He said that this sin was forgiven them.) But as to your words touching the Spirit, they will have no excuse. For in proof that He is speaking of what was said of Him before the crucifixion, He added, “Whosoever shall speak a word against the Son of Man, it shall be forgiven him; but whosoever shall speak against the Holy Ghost,” there is no more forgiveness. Wherefore? Because this is known to you; and the truths are notorious which you harden yourselves against. For though ye say that you know not me; yet of this surely you are not ignorant, that to cast out devils, and to do cures, is a work of the Holy Ghost. It is not then I only whom you are insulting, but the Holy Ghost also. Wherefore your punishment can be averted by no prayers, neither here nor there.
For so of men, some are punished both here, and there, some here only, some there only, others neither here nor there. Here and there, as these very men (for both here did they pay a penalty, when they suffered those incurable ills at the taking of their city, and there shall they undergo a very grievous one), as the inhabitants of Sodom; as many others. There only, as the rich man who endured the flames, and had not at his command so much as a drop of water. Here, as he that had committed fornication among the Corinthians. Neither here nor there, as the apostles, as the prophets, as the blessed Job; for their sufferings were not surely in the way of punishment, but as contests and wrestlings.
Let us labor, therefore, to be of the same part with these: or if not with these, at least with them that wash away their sins here. For fearful indeed is that other judgment, and inexorable the vengeance, and incurable the punishment.
Source: Homilies on the Gospel of St. Matthew (New Advent)