20 Against this gratuitous gift, against this grace of God, does the impenitent heart speak. This impenitence then is “the blasphemy of the Spirit, which shall not be forgiven, neither in this world, neither in the world to come.” For against the Holy Spirit, by whom they whose sins are all forgiven are baptized, and whom the Church has received, that “whosesoever sins she remits, they may be remitted,” does he speak, whether in the thought only, or also in the tongue, a very heinous and exceedingly ungodly word, who “when the patience of God leads him to repentance, after his hardness and impenitent heart treasures up unto himself wrath against the day of wrath, and revelation of the righteous judgment of God, who will render to every man according to his deeds.” This impenitence then, for so by some one general name may we call both this blasphemy and the word against the Holy Ghost which has no forgiveness for ever; this impenitence, I say, against which both the herald and the Judge cried out, saying, “Repent ye, for the kingdom of heaven is at hand;” against which the Lord first opened the mouth of the Gospel preaching, and against which He foretold that the same Gospel was to be preached in all the world, when He said to His disciples after His resurrection from the dead, “it behooved Christ to suffer, and to rise from the dead the third day, and that repentance and remission of sins should be preached in His Name among all nations, beginning at Jerusalem:” this impenitence, in one word, has no forgiveness “neither in this world, nor in the world to come;” for that repentance only obtains forgiveness in this world, that it may have its effect in the world to come.
Source: Sermons on Selected Lessons of the New Testament (New Advent)