What is the sin against the Holy Spirit?
The Evangelists, Matthew (12:30-32), Mark (3:28-29), and Luke (12:10), gave summarized but sufficient accounts of Jesus’s explanation of the unforgivable sin against the Holy Spirit, the minimum necessary for the purpose of bringing people to belief and salvation. On September 2nd, 1945, Jesus showed Maria Valtorta this same scene in a vision, which she described at length as a first-hand witness. Jesus, through Maria, His instrument, or “pen”, brings this Gospel scene to life, helps one to understand it more clearly and live it out better. I’ve extracted, copied, and pasted His explanation in full from The Poem of the Man-God: Vol. 2 (ch. 268, p. 434) below:
Do you not know that there will be a century that will have a beginning but no end and that it will be Mine? I shall triumph in it gathering round Me its children and they will live forever like the century that I shall have created and I am already creating it, giving the spirit its true value above the flesh, the world, and above the infernal angels whom I expel because I can do everything. That is why I say that those who are not with Me are against Me, and those who do not gather with Me, scatter. Because I am He Who I am. And he who does not believe that, which was already prophesied, sins against the Holy Spirit, Whose Word was announced by the prophets, and it is neither false nor wrong, and must be believed without resistance.
And I tell you: men will be forgiven everything, all their sins and their blasphemy. Because God knows that man is not only spirit, but also flesh and his flesh, when tempted, is subject to sudden weakness. But blasphemy against the Spirit will not be forgiven. He who has spoken against the Son of Man will still be forgiven, because the weight of the flesh enveloping My Person and the man who speaks against Me, can still mislead. But he who has spoken against the Holy Spirit will not be forgiven, either in this or in future life, because the Truth is what it is: clear, holy, undeniable and manifested to the spirit in such a way that it cannot mislead. Only those who err deliberately want to err. To deny the Truth spoken by the Holy Spirit is to deny the Word of God and the Love given by that Word for the sake of men. And the sin against Love is not forgiven. (cf. Vol. 4, chs. 419, 485)