Here is a good exegesis on the unforgivable sin and Hebrews 6. I'll link the whole article, and copy the relevant paragraphs here.
"Hebrews 6:1-6 reads like this:
"
1 Therefore let us leave the elementary teachings of Christ and go on to maturity, not laying again the foundation of repentance from acts that lead to death, and of faith in God, 2 instruction about baptisms, the laying on of hands, the resurrection of the dead, and eternal judgment. 3 And God permitting, we will do so.
"4 It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, 5 who have tasted the goodness of the word of God and the powers of the coming age, 6 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."
The first two verses tell us about "the elementary teachings of Christ"—that is, the basic truths of the Christian faith. This is important because it will set us up for the discussion of apostasy.
Note that they walk us through an ordo salutis—the stages of the Christian life: repentance, faith, baptism, laying on of hands (i.e., confirmation), resurrection, and judgment. Two truths preceding Christian initiation (repentance and faith), two truths at initiation (baptism and confirmation), and two truths at the end of the Christian life (resurrection and judgment).
The author says he won't go over the basic teachings of Christ again because it is impossible to renew to repentance those who have fallen away. This is often a very problematic verse (especially for those who believe it is impossible to lose one's salvation), and is often thought to pertain to the unforgivable sin. However, this is not the case.
To see why, we must first eliminate a dodge that is often used to render this verse a counterfactual hypothetical. As it appears in many English translations, v. 6 is often opened with the clause "if they fall away." However, this is not an accurate rendering of the Greek text, as even eternal securitists (such as Kendall) will admit. The Greek is simply kai parapesontas, which of course means "and (kai) have fallen away (parapesontas)"—parapesontas being an aorist—just like in the other four clauses in the preceding two verses, of which this clause is the final link in the chain of parallel aorist clauses identifying the apostates. The passage, correctly translated, thus reads:
"It is impossible for those who (a) have once been enlightened, (b) have tasted the heavenly gift, (c) and have been made partakers in the Holy Spirit, (d) and have tasted the goodness of the word of God and the powers of the coming age, (e) and have fallen away, to be brought back to repentance, because to their loss they are re-crucifying the Son of God and subjecting him to public disgrace."
Or more shortly:
"It is impossible for those who have once been enlightened . . . and have fallen away, to be brought back to repentance, because to their loss they are re-crucifying the Son of God and subjecting him to public disgrace."
The Greek of the passage presents the falling away as an accomplished fact, not a hypothetical possibility. (Thus an eternal securitist would have to say that they were never inwardly a Christian to begin with, only outwardly.)
Nevertheless, the passage does not pertain to the unforgivable sin. Many have misread the passage, being misled by the hypothetical ("if . . .") translation of v 6, and have argued: "If a person did fall away then they could not come back because they would have to re-crucify Christ, and that is impossible since he died only once!"
But this is simply not what the passage says. It does not say that if one tried to come back one would have to re-crucify Christ. It does not present the re-crucifixion as something that would need to happen if someone came back. It presents the re-crucifixion as a present reality. Just read the text: "because to their loss they are re-crucifying [present tense, active voice in both Greek and English] the Son of God and subjecting him to public disgrace." The text says that the apostates are re-crucifying Christ now, not that they would need to if they came back.
This is where understanding the Jewish context (and content) of the letter is so important. By returning to Judaism, the apostates are declaring that Jesus was a false Messiah (else they would not leave faith in him as the true Messiah). But by declaring Jesus to be a false Messiah, they are declaring that he deserved what he got when he was crucified—because it is axiomatic that every false Messiah deserves death and public humiliation. They, like the fox in Aesop's fable "The fox and the grapes," are having an attack of sour grapes and were running around saying: "Well, he wasn't the real Messiah. He deserved what he got. He deserved to be crucified and put to public humiliation. As it says in the Torah, 'Cursed is every man who is hung upon a tree!'"
Thus the re-crucifixion and humiliation of Christ was something the apostates were doing while they were maintaining their rebellion against the Messiah they had once accepted. This indicates an enormous hardness of heart, which is why the author tells us, "It is impossible for those . . . to be brought back to repentance." The hardness of their hearts prevents it.
This is, of course, a practical rule rather than a dogmatic (absolute) rule. Because of the hardness of heart the Jewish apostates are displaying by publicly denouncing Jesus, declaring that he deserved crucifixion and humiliation, it is as a practical matter impossible to renew them to repentance and faith in Christ. This does not in any way mean it is an absolute impossibility to renew them to repentance, for "With men it is impossible, but not with God; for all things are possible with God" (Mark. 10:27)."
https://www.ewtn.com/library/ANSWERS/UNFORGIV.HTM
(Coloring and bolding of the text, not in original article)
