"He that shall blaspheme against the Holy Ghost hath never forgiveness." Mark 3:29 The doctrine of the Trinity defines the Father, the Son and the Holy Spirit---the one true God. Jesus Christ the Son is God incarnate, the Truth, the Way and the Life, for life is of the living triune God and no other.
The Holy Spirit is associated with the triune God's divine actions, such as creation, judgment, prophecy, cleansing, sanctioning, and justifying:
Matthew 16:17--And Jesus answered and said unto him, Blessed art thou, Simon Barjona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven.
Mark 12:36---For David himself said by the Holy Ghost, The LORD said to my Lord, Sit thou on my right hand, till I make thine enemies thy footstool.
Luke 1:67--And his father Zacharias was filled with the Holy Ghost, and prophesied...
1 Cor. 6:11---And such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God.
John 16:13---Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will show you things to come.
John 14:26---But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.
John 15:26-- But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me...
Is. 8:11-- For the LORD spake thus to me with a strong hand, and instructed me that I should not walk in the way of this people...
Jer. 15:17--I sat not in the assembly of the mockers, nor rejoiced; I sat alone because of thy hand: for thou hast filled me with indignation.
1 Tim. 4:1---Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils
Romans 8:9-11--- But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his. (The Holy Spirit and the Trinity, tektonics.org)
Before proceeding, we must clearly understand what the unforgivable blasphemy is not.
Scripture reveals that the triune God knows your spirit (heart) better than anyone including yourself:
"God, who knows the heart, showed that he accepted them by giving the Holy Spirit to them, just as he did to us" (Acts 15:8)
This means for example, that if at one time you, like Job, felt great anger against the Holy Spirit and maybe accused Him of causing you to suffer and perhaps you even turned away from Him for a time, Jesus's Blood nevertheless atones for you because God Who knows your heart knows you really love Him and that you didn't mean it (Acts 13:39):
"And so I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven. Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come." (Matthew 12:31-34)
"For if you confess with your mouth, Jesus is Lord, and believe in your heart that God raised him from the dead, you will be saved. For it is with your heart that you believe and are justified, and it is with your mouth that you confess and are saved." (Romans 10:9-10)
And now to the unforgivable blasphemy:
"Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come." (Matthew 12:31-34)
By "blaspheming" the Holy Spirit, Jesus refers to all who deny the divine authority and activity of the Spirit (the triune God's divine actions). This also applies to those who refuse the prodding of the Holy Spirit to believe the Gospel. They thus blaspheme the Holy Spirit by putting his activity down to guilt feelings, for example.
Using the Pharisees as an example, Jesus Christ speaks of a spirit (heart and/or mind, will, conscience) so hardened and unrepentant that it is capable of accusing the Son of having Satan in Himself. Such a spirit is guilty of blasphemy against the Holy Spirit:
"But because of your stubbornness and your unrepentant heart, you are storing up wrath against yourself for the day of God's wrath, when his righteous judgment will be revealed" (Romans 2:5).
Hebrews 6:4-6 exemplifies the blasphemy of Karl Marx, the father of the devilish Communist Manifesto:
"It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, who have tasted the goodness of the word of God and the powers of the coming age, if they fall away, to be brought back to repentance, because to their loss they are crucifying the Son of God all over again and subjecting him to public disgrace"
In his youth Karl Marx professed to be a Christian. In his work, "The Union of the Faithful with Christ," we read,
"Through love of Christ we turn our hearts at the same time toward our brethren who are inwardly bound to us and for whom He gave Himself in sacrifice." (Marx & Satan, Richard Wurmbrand, p. 11)
In his thesis, "Considerations of a Young Man on Choosing His Career," Marx writes that Christianity teaches that Jesus Christ sacrificed Himself for all men and He is the Ideal toward which all men strive:
"...and who shall dare contradict such claims? If we have chosen the position in which we can accomplish the most for Him, then we can never be crushed by burdens, because they are only sacrifices made for the sake of all." (ibid, p. 11)
Marx began life as a self-professed Christian but shortly after receiving his graduation certificate he rejected the leading of the Holy Spirit, thus a new, darkly frightening Marx began to emerge.
In his poem, "Invocation of One in Despair, " he declared his intention to avenge himself against the "One who rules above..." Like his father the devil Marx brazenly states:
"I shall build my throne high overhead..." (ibid, p. 12)
Marx's boast sounds much like Lucifer's:
"I will ascend into heaven, I will exalt my throne above the stars of God." (Isaiah 14:13)
Like satan, Marx now dreamt of ruining the world created by the living, triune God:
"Then I will be able to walk triumphantly, Like a god, through the ruins of their kingdom. Every word of mine is fire and action. My breast is equal to that of the Creator." (Quoted in Deutsche Tagespost, West Germany, Dec. 1982)
In the poem called "The Player," Marx confesses that a spiritual transformation, or counter-conversion has occurred:
"The hellish vapors rise and fill the brain, Till I go mad and my heart is utterly changed. See this sword? The prince of darkness Sold it to me. For me he beats the time and gives the signs. Ever more boldly I play the dance of death." (ibid, Wurmbrand, p. 15)
When in his rebellion against the triune God Marx found himself compelled to negate the central conclusion of the Genesis account of creation ex nihilo because of his desire to establish that instead of man created in the spiritual image of God, man "owes his existence (only) to man" and to thereby prove that the entire history of the world is the creation of man, he virtually recapitulated Thomas Aquinas' argument for the existence of the supernatural triune God. (Political Apocalypse, Ellis Sandoz, pp. 113-114)
Marx first admits that man can only,
"...consider himself independent when (he) owes his existence to himself. A man who lives by the grace of another regards himself as a dependent being. But if I live completely by the grace of another..I owe him not only the maintenance of my life, but if he has, moreover, created my life (then) he is the source of my life. When it is not of my own creation, my life has necessarily a source of this kind outside of it. The creation is therefore an idea very difficult to dislodge from popular consciousness." (ibid)
From this intellectual exercise Marx correctly concludes that the idea that nature and man are not created, (do not exist in their own account and/or were spontaneously generated from nothing) is incomprehensible because it "contradicts everything tangible in practical life."
Marx knows that the supernatural triune God exists, so he chooses to will himself to 'know God does not exist' and boldly prohibits the asking of questions pertaining to Gods' existence. To even ask if God exists becomes wholly unreasonable, hence reasonable men do not ask such questions. Even to deny the existence of God (atheism) is unreasonable. It's best to simply affirm socialist reality.
In this light we can understand that Hebrews 6:4-6 refers to an individual who had by the leading of the Holy Spirit tasted the goodness of the word of God, now willfully chooses to renounce his faith and close his heart to the Holy Spirit.
Such a heart is inward-turned, tightly closed and so thoroughly hardened and unrepentant--and in the case of Marx demonically occluded--that it not only declares war against God but goes so far as to prohibit questions about His existence and actively seek the loss of salvation for others. Having willfully rejected eternal paradise it wants no others to have it either.
It is too late for such a soul. It will not be brought back to repentance. By its' own decision it decided in favor of hell-- outer darkness---for all eternity.
In conclusion, the unpardonable blasphemy (sin) is this: willful unbelief.