7 Some think that they only sin against the Holy Ghost, who having been washed in the laver of regeneration in the Church, and having received the Holy Spirit, as though unthankful for so great a gift of the Saviour, have plunged themselves afterwards into any deadly sin; as adultery, or murder, or an absolute apostasy, either altogether from the Christian name, or from the Catholic Church. But how this sense of it may be proved, I know not; since the place of repentance is not denied in the Church to any sins whatever; and the Apostle says that heretics themselves are to be reproved to this end, “If God perhaps will give them repentance to the acknowledging of the truth; And that they may recover themselves out of the snare of the devil, who are taken captive by him at his will.” For what is the advantage of amendment without any hope of forgiveness?
Finally, The Lord did not say, “the baptized Catholic who shall speak a word against the Holy Ghost;” but “he who,” that is whosoever speaks, be he who he may, “it shall not be forgiven him, neither in this world, neither in the world to come.” Whether then he be a heathen, or a Jew, or a Christian, or a heretic from among Jews or Christians, or whatsoever other title of error he have, it is not said, this man, or that man; but “whosoever speaks a word against the Holy Ghost,” that is who blasphemes the Holy Ghost, “it shall not be forgiven him, neither in this world, neither in the world to come.”
But moreover if every error contrary to truth, and inimical to Christian peace, as we have shown before, “speaks a word against the Holy Ghost;” and yet the Church does not cease to reform and gather out of every error those who shall receive remission of sins, and the Holy Ghost Himself, whom they have blasphemed; I think I have discovered an important secret for the clearing up this so great a question. Let us seek then from the Lord the light of explanation.
Source: Sermons on Selected Lessons of the New Testament (New Advent)