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.
Here is the point: Jesus is the only one who said this term and spoke about this sin. Everyone else has to speculate in the Bible where else it may be said or give an opinion as to what they think it is in addition to what Jesus said it is.
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.