22 And let it not seem absurd, that whereas a man who perseveres in hardened impenitence even to the end of this life, speaks long and much against this grace of the Holy Spirit; yet the Gospel has called this so long contradiction of an impenitent heart, as though it were something of short duration, “a word,” saying, “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.”
For though this blasphemy be long continued, and made up of, and drawn out at length in very many words, yet it is the manner of Scripture to call even many words “a word.” For no prophet ever spoke one word only; yet we read, “the word which came to such and such a prophet.” And the Apostle says, “Let the elders be counted worthy of double honour, especially they who labour in the word and doctrine.” He does not say, “in words,” but, “in the word.” And St. James, “Be doers of the word, and not hearers only.” He again does not say, “of the words,” but, “of the word;” although so many words out of the Holy Scriptures are read, and spoken, and heard in the Church at her celebrations and solemnities.
As therefore, how long a time soever any of us have laboured in preaching the Gospel, he is not called a preacher of the words, but of the word; and how long time soever any of you may have attentively and diligently listened to our preaching, he is called a most earnest “hearer” not of the words, but “of the word;” so after the style of the Scripture and the custom of the Church, whoso throughout his whole life in the flesh, to whatever length it may be extended, shall have spoken no matter how many words, whether by mouth, or the thought only with an impenitent heart, against that remission of sins which is granted in the Church, he speaks “a word” against the Holy Ghost.
Source: Sermons on Selected Lessons of the New Testament (New Advent)