(2) "It is impossible to restore again to repentance those who once been enlightened and have
tasted the heavenly gift, and have become partners (Greek: "metachoi") with the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come, and then have fallen away, since on their own, they are crucifying again the Son of God and are holding Him up for contempt (6:4-5)."
Such people do not benefit from eternal security. In the context the author likens them to ground that grows "thorns and thistles" that is "cursed" and destined to being "burned over (6:8)" and therefore to be consigned to Hell. Yet these apostates have received the "heavenly gift" of salvation (cp.
Ephesians 2:8-9) and the Holy Spirit, and so, they were once born again of the Spirit.
The claim that those who become “metachoi” (= “partners” or “sharers” with the Holy Spirit might not actually possess the Holy Spirit can be dismissed on 2 grounds: (a) The author addresses his readers as "holy partners ("metachoi") in the heavenly calling" (3:1). (b) The author then warns: "For we have become partners ("metochoi") of Christ,
if only we hold our FIRST confidence firm till the end (3:14)." By implication the saving partnership with Christ or the Holy Spirit is dissolved if we don't persevere until the end, but become apostates! "Metachoi" in 6:4 means "partners" as well as "sharers." So the Christians addressed in 6:4 "have become partners ("metochoi") with the Holy Spirit," meaning they have been saved by the work of the Spirit. The significance of the phrase, "it is impossible
to restore again to repentance" must not be overlooked. "Restore
again" implies the Christians in question have already genuinely repented when they became partners with Christ and the Holy Spirit. This is the standard interpretation of academic book commentaries on Hebrews, which posters here have never read. By academic book commentaries, I mean commentararies on just one Bible book written by PhDs in Scriptures who know and discuss in detail the nuances of the Greek text. For example, justbyfaith likes to cite John Courson, who is a pastor/ preacher, not a scholar qualified to discuss the Greek text in detail and who has not written ab an academic commentary focused on just one NT book.