So the sin of blasphemy against the Holy Spirit is forgivable. It is impossible for the author to bring to repentance "those who have experienced the good things of heaven and shared in the Holy Spirit, 5 who have tasted the goodness of the word of God and the power of the age to come— 6 and who then turn away from God. " But with God nothing is impossible.
I have seen people who were under the influence of drugs and alcohol blasphemy the Holy Spirit. I heard atheists blasphemy the Holy Spirit. Yet they were saved and now follow The Lord. It is a sin of knowledge and can't be done in ignorance. That is the point.