Everyone who speeks a word against The Son Of Man will be forgiven, but...
anyone who blasphemes aigainst The Holy Spirit will not be forgiven.
But whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin.
Blasphemy is irreverence toward something. Reverence is being in awe of something. So if you have at one point not been in awe of The Holy Spirit, then your soul is lost. That means that merely considering the possibility that The Holy Spirit doesn't exsist, is an eternal sin.