The Holy Spirit is the One who witnesses and gives confirmation to every person that Jesus is the Messiah and the only One from whom we receive salvation.
All sins capable of being committed are forgiven EXCEPT ONE (the unforgivable sin) — which is REJECTING THE WITNESS of the Holy Spirit in affirming that Jesus is the Messiah of Israel and Savior for all who accept Him as such.
Hence REJECTING the Holy Spirit and His witness of Jesus the Messiah (and His is the ONLY WITNESS that is authoritative and inerrant, not any person’s witness) - is rejecting the salvation that comes from the Messiah of Israel - and IS PERMANENT once one dies. It’s UNFORGIVABLE.
Therefore do NOT DENY the witness of the Holy Spirit and hence, engage in the UNFORGIVABLE SIN!