23 Therefore not only every word spoken against the Son of Man, but, in fact, every sin and blasphemy shall be forgiven unto men; because where there is not this sin of an impenitent heart against the Holy Ghost, by whom sins are remitted in the Church, all other sins are forgiven. But how shall that sin be forgiven, which hinders the forgiveness of other sins also? All sins then are forgiven to them in whom is not this sin, which shall never be forgiven; but to him in whom it is, since this sin is never forgiven, neither are other sins forgiven; because the remission of all is hindered by the bond of this one.
It is not then that “whosoever speaks a word against the Son of Man shall be forgiven,” but “whoso speaks against the Holy Ghost shall not be forgiven,” for that in the Trinity the Holy Ghost is greater than the Son, which no heretic even has ever maintained; but since whosoever he be that resists the truth and blasphemes the Truth, which is Christ, even after such a manifestation of Himself among men, as that the Word who is the Son of Man and very Christ, “became flesh and dwelt among us;” if he have not also spoken that word of the impenitent heart against the Holy Ghost, of whom it is said, “Unless a man be born of water and of the Spirit;” and again, “Receive the Holy Ghost; whosesoever sins ye remit they are remitted unto them;” that is, if he shall repent, he shall thereby receive the gift of the remission of all his sins, and of this also, that he “has spoken a word against the Son of Man,” because to the sin of ignorance, or obstinacy, or blasphemy of whatever kind, he has not added the sin of impenitence against the gift of God, and the grace of regeneration or reconciliation, which is conferred in the Church by the Holy Spirit.
Source: Sermons on Selected Lessons of the New Testament (New Advent)