34 Whosoever therefore shall be guilty of impenitence against the Spirit, in whom the unity and fellowship of the communion of the Church is gathered together, shall never have forgiveness; because he has stopped the source of forgiveness against himself, and deservedly shall he be condemned with the spirit, which is divided against himself, who is himself also divided against the Holy Spirit which is not divided against Himself. And of this the very testimonies of the Gospel warn us, would we with good attention search them.
For according to Luke the Lord does not say, “That he who blasphemes against the Holy Ghost shall not be forgiven:” in that place where He is answering those who said that He cast out devils by the prince of the devils. Whence it would seem that this was not said once only by the Lord; but we must not carelessly pass over the consideration of the occasion on which this last also was spoken. For He was speaking of those who should have confessed or denied Him before men, when He said, “Also I say unto you, Whosoever shall confess Me before men, him shall the Son of Man also confess before the Angels of God.
But he that denies Me before men, shall be denied before the angels of God.” And lest from this the salvation of the Apostle Peter should be despaired of, he immediately subjoined, “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; blasphemes,” that is, with that blasphemy of an impenitent heart, by which resistance is made to remission of sins which is granted in the Church by the Holy Ghost.
And this blasphemy Peter had not, who presently repented, when “he wept bitterly,” and who after he had overcome the spirit who is divided against himself, and who had desired to “have him to harass him,” and against whom the “Lord prayed for him that his faith might not fail,” even received the Very Holy Spirit whom he resisted not, that not only his sin might be forgiven him, but that through him remission of sins might be preached and dispensed.
Source: Sermons on Selected Lessons of the New Testament (New Advent)