The one unpardonable, "unforgivable" . . . eternal . . . sin . . . is the persistent, ongoing refusal and rejection of Jesus, despite the ample evidence as to His divinity in the ways of signs, fulfilled prophecy, witness testimony, and especially the conviction of the conscience--of the Holy Spirit Who testifies to the truth of His claim and validity of the Word of God . . . .
I'm sure there are better explanations . . . but I am confident that this generally well describes it. It is not a blasphemous word uttered in momentary passion . . . nor is it when people don't accept the self-asserted prophecies of someone claiming to speak messages from the Holy Spirit . . . ("You are blaspheming the Holy Spirit if you do not follow me!")
There is certain hardness of heart . . . which simply will not deign--is apparently unable--to be broached by the Self evident truth of God, the history of Christ and the prophets . . . the self sacrificing fact of convicted believers . . . and the witness within . . . .
It is a determined, endless rebuff--a decision to live and die as a proud rebel . . . and so, the one who digs in against God . . . continues into their own haunted eternity . . . for there is no post-mortem plan of salvation, contrary to what some wishful (secretly rebellious) thinkers like to imagine . . . .
At the same time there is constant hope and possibility while still in this life . . . . The "penitent thief" who, in a matter of hours hanging there on his cross next to Jesus . . . was able to grasp the gravity of The Situation . . . and come to saving grace . . . to gain the promise of life eternal in paradise . . . .
The mode of true repentance was there, which was sufficient for his salvation; not years of good works . . . the obeying of rules and law . . . tithes . . . time spent in church . . . theological mastery . . . secret handshakes, mystical phrases . . . years of asceticism . . . clean living . . . dietary proscriptions . . . sacrifices, alms, a life of charity . . . .
Well, we don't know how many, if any of such religious observances and practices he might have kept in his life . . . probably few as crucifixion was reserved for the worst kinds criminal whose deeds were notorious. But we do know that even if he had kept them all . . . it still wasn't enough to enter the Kingdom of God . . . .
What then, was required? When the other criminal there hurled insults at Jesus, the penitent thief asked, "Don't you fear God? Since you are under the same condemnation?"
A holy, reverent, sincere fear of God was the first step shown in his transformation from unsaved to saved.
Then the realization of his own wickedness, accepting that he was due the punishment he was getting: "We are punished justly, for we are getting what our deeds deserve."
Many people simply cannot accept and do not appreciate . . . that . . . we are born sinners who, in our fallen condition, hate eternal God and thus deserve eternal punishment . . . .
"But this man has done nothing wrong," said the thief further, acknowledging the innocence of Jesus.
Next, we see that the thief had faith . . . that Jesus was Lord and had the authority and power to save him, as he called him "Lord," . . . .
And then prayed, in essence, asking forgiveness, beseechingly, "remember me when you come into your Kingdom," and showing humility in the plea. He did not recount great works or assert worthiness.
On the contrary, he had just admitted to being an unworthy criminal, deserving punishment . . . but, believing that Jesus was Lord, asked only to be "remembered" . . . .
Jesus assured him this was enough. "Today, you will be with me in paradise."
How different this lowly, harrowing exchange--and type of a degraded man finding God . . . compared to those who vaunt theological knowledge or stand proudly in self-made, glittering robes of "good works" . . . or walk through halls and corridors with their airs of piety . . . .
And how different from those who don't even bother to entertain the possibility of a just God, but instead scorn believers as weak and foolish, stonily rejecting the evidence and example of God in the flesh, to their bitter, angry, defiant end . . . .
In fact, the two immediate examples above--one in religious garb, the other a so-called "free thinker"--BOTH share the same unpardonable rebellion, should they never, in this life, like the thief on the cross, accept their true plight . . . .
How true it is that, "God has chosen the foolish things of the world to confound the wise; and God has chosen the weak things of the world to confound the things which are mighty" . . . .