Heb 6:4-6 context About rejection/unbelief, not about losing salvation.
To begin with; here are just 7 of many scriptures. That make it abundantly clear that salvation is eternal. (Jn 3:16 & 6:47; Jn 10:27-29; Rom 8:35; 38-39 & 10:9-13; Phil 1:6)
Some background: 1st who is the book written to? Hebrews. The main theme throughout the book of Hebrews is NEW ""BETTER"" COVENANT.
Christ is better: Better than angels, because they worship the Christ. Better than Moses, because He created Him. Better than the Aaronic priesthood, because Christ's sin sacrifice is once for all time. Better than the law, because Christ mediates a better blood covenant.
These Jews who were having a hard time making a break from total indoctrination in a legalistic religion. Judaism's temple worship, daily sacrifices & personal sin sacrifices. Many are reversing course and choose to continue in their religion to avoid persecution, often being totally ostracized.
Background 2: The parable of the sower Matt:13 & Mk 4:5
4 Jesus said, a sower sowed, some seeds fell by the wayside, the fowls came & devoured them
5 Some fell upon stony places, they had not much earth: they sprung up, because they had no deepness of earth:
6 when the sun was up, they were scorched; because they had no root, they withered away
7 And some fell among thorns; the thorns sprung up, & choked them
8 Other fell into good ground, & brought forth fruit, some a hundredfold, some 60-fold, some 30-fold
Background 3: Pro 26:11 & 2 Pet 2:20-22 "A dog goes back to its vomit"
Now let's parse Heb 6:4 (A) For it is impossible for those who were once enlightened.
(MY NOTE: Enlightened definition: having been showed a rational well-informed outlook. Just because a person has been shown how to get someplace. Doesn't mean they will choose to go there)
Heb 6:4 (B) For it is impossible for those who have "tasted" the good Word of God,
(MY NOTE: To taste something: Means just that, "TASTE IT" not consume or accept the whole meal. But, to "TASTE". And now you can decide if you want to finish eating it or not)
Heb 6:4 (C) Were made partakers of the Holy Spirit
(MY NOTE: Jn 12:32 And I, if I be lifted up from the earth, will draw all men unto me.
(MY NOTE: Jesus was in fact lifted up on a cross & ultimately from the grave. Heb is written post resurrection. He/His Holy Spirit drew these Jews via His Holy Spirit & they rejected Jesus's sin atonement by returning to Temple sacrifices)
Ok, Jesus sowed the Word (they TASTED), the Holy Spirit confirmed (enlightened) the truth into their hearts
After TASTING the WORD of God. Like the seeds that fell by the wayside onto stony ground of were choked by thorns/Pharisees. They rejected God's Grace/Word (Jesus Christ)
Again, Heb 6:4-6 scriptures are about the price of rejection & UNBELIEF! NOT losing salvation!
Under the old covenant (Jews only) the only sin that couldn't be covered was, IDOLATRY
Under the new covenant (covers all believers) the only unforgivable sin is, UNBELIEF!
Conclusion:
These Jews rejected Christ; God's saving grace accessed thru Faith (Rom 5:2). They NEVER were SAVED. After HEARING & TASTING the WORD of ENLIGHTENMENT they REJECTED it = UNBELIEF. And returned to their legalistic traditions/the law. Most notably animal sacrifices!
1 Jn 2:19 They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.