Jesus said, "Truly I say to you, all sins shall be forgiven the sons of men, & whatever blasphemies they utter; but whoever blasphemes against the Holy Spirit, never has forgive- ness, but is guilty of an eternal sin" -------because they were saying 'He [Jesus] has an unclean spirit.'
In the above passage, the Lord clearly defined the only sin which 'never hasforgiveness'. It is when a person know- ingly & conciously blasphemes or curses the Holy Spirit of God.
Undoubtedly, this would have to be a person who has already heard & understood the story of Jesus dying on the cross in Jerusalem as Savior and Lord, PLUS has ALSO heard & understood about eternal life being offered to that person by receiving the gift of the Holy Spirit.
Then, for that person [who knows all of this] to conciously & knowingly 'curse or blaspheme the Holy Spirit',while firmly rejecting the Spirit of God from ever coming into his own life, has then committed the sin which 'never has forgiveness'.
This particular profile of person will have, knowingly, sealed his own eternal destiny in his own lifetime, dooming himselfto eternal separation from God's presence.