13 I see that you are now wishing to hear, since it is not every blasphemy of the Spirit, what that blasphemy is which shall not be forgiven, and what that word is, since it is not every word which if it shall be spoken against the Holy Ghost, shall not be forgiven neither in this world, neither in the world to come. And for my part I should be willing to tell you at once, what you are so very intently waiting to hear; but bear for a while the delay which a more careful diligence requires, till by the Lord's assistance I shall unfold the whole meaning of the passage before us.
Now the other two Evangelists, Mark and Luke, when they spoke of the same thing, did not say “blasphemy” or “a word,” that we might understand it not of every blasphemy, but of some sort of blasphemy; not every word, but some certain word. What then did they say? In Mark it is thus written, “Verily I say unto you, all sins shall be forgiven unto the sons of men, and blasphemies, where-withsoever they shall blaspheme. But he that shall blaspheme against the Holy Ghost, has never forgiveness, but shall be held guilty of an eternal offense.” In Luke it is thus: “And whosoever shall speak a word against the Son of Man, it shall be forgiven him; but unto him that blasphemes against the Holy Ghost, it shall not be forgiven.” Is there any departure from the truth of the same proposition because of some diversity in the expression?
For indeed there is no other reason why the Evangelists do not relate the same things in the same way, but that we may learn thereby to prefer things to words, not words to things, and to seek for nothing else in the speaker, but for his intention, to convey which only the words are used. For what real difference is there whether it is said, “Blasphemy of the Spirit shall not be forgiven;” or “he that blasphemes against the Holy Ghost, it shall not be forgiven him.” Except perhaps that the same thing is declared more plainly in this last than in the other form; and so one Evangelist does not overthrow, but explains the other.
Now “blasphemy of the Spirit” is an unevident expression; because it is not directly said what spirit; for every spirit is not the Holy Spirit. Thus it might be called “blasphemy of the spirit,” when a man blasphemes with the spirit; as that may be called “prayer of the spirit,” when one prays with the spirit. Whence the Apostle says, “I will pray with the spirit, and I will pray with the understanding also.” But when it is said, “he that shall blaspheme against the Holy Ghost,” these ambiguities are removed.
So the expression, “has never forgiveness, but shall be held guilty of an eternal offense;” what is it, but what according to Matthew is expressed, “it shall not be forgiven him, neither in this world, neither in the world to come”? The very same idea is expressed in different words and different forms of speech. And what is in Matthew, “Whosoever speaks a word against the Holy Ghost,” that we might not understand it of anything but blasphemy, others have more clearly expressed, “He that shall blaspheme against the Holy Ghost.” Yet the same thing is said by all; nor did any one of them depart from the intention of the Speaker, for the sake of understanding which only are words spoken, and written, and read, and heard.
Source: Sermons on Selected Lessons of the New Testament (New Advent)