Let me repeat some of what I said before about burning, as I feel this is a key reason why people see those in verses 4-6 as unbelievers - merely professing faith. The burning of a field that yields thorns and thistles does not destroy the field. The purpose is to hopefully prepare it to yield a useful crop in the future. The parallels with 1 Corinthians 3:10-15 are so close as to be undeniable and 1 Corinthians 3 clearly refers to lost rewards and a painful experience...
1 Corininthians 3:15 - "but he will be saved; yet it will be like an escape through fire."
Now, compare that with:
Hebrews 6:8 - "it is worthless and close to being cursed, and it ends up being burned."
Burning is often used in the NT to refer to God's discipline and purging:
1 Peter 1:6, 7 - In this you greatly rejoice, even though now for a little while, if necessary, you have been distressed by various trials, so that the proof of your faith, being more precious than gold which is perishable, even though tested by fire, may be found to result in praise and glory and honor at the revelation of Jesus Christ;
(Compare with James 1 below: )
James 1:2-4 - Consider it all joy, my brethren, when you encounter various trials, knowing that the testing of your faith produces endurance. And let endurance have its perfect result, so that you may be perfect and complete, lacking in nothing.
1 Corinthians 3:13 - each man's work will become evident; for the day will show it because it is to be revealed with fire, and the fire itself will test the quality of each man's work.
Revelation 3:18 - I advise you to buy from Me gold refined in the fire so that you may be rich, and white clothes so that you may be dressed and your shameful nakedness not be exposed, and ointment to spread on your eyes so that you may see.
You've made some good observations and a common point made in defense of those not being believers is about the apparent pronouns, which you're alluding to. But the issue for it being believers is much more complex than just the pronouns, and seeing a change in the audience at vs. 9, and is difficult to refute.
Since I believe that we are eternally secure, I could deal with it either way that fit the exegesis, of course, those being either believers or unbelievers. The reason I see it the way I do is purely based on the text and its context. My theology does not pressure me to view it either way. (Nor does it pressure Sproul apparently, as well.)
Some refer to the pronouns in verses 4 - 6 as indicating a change in readership from verse 9, but actually, in verses 4 and 6 the pronouns "those," they", and "them" don't really exist... There is a string of participles which are more literally translated as "having..." or "while..." etc. or as "who have..."
Also actually in vs. 4 the phrase translated something like "For in the case of those who have once been enlightened..." is really something like "For [it is] impossible [for] ones once enlightened..." in a more wooden translation.
Similarly, in vs. 6 what is typically translated as "and then have fallen away, it is impossible to renew them again to repentance (act. from vs. 4) since they crucify to themselves again the Son of God..." would be translated more woodenly as "and falling away again to renew to repentance crucifying to/for themselves (
ἑαυτοῖς - plural reflexive pronoun - 'themselves') again the Son of God..."
The only pronoun is a reflexive one. But translating participles so that it reads somewhat like normal English requires the use of pronouns to make it clear. That's why they were "inserted."
But IMO the author was speaking to some believers who were not going on to maturity (5:11-6:3). He speaks aboutthe serious immatrue condition of some of his readership, then after describing the consequences to them he goes on to say starting in vs. 9 that he has confidence that they will not end up like that.
The warning doesn't make any sense if he is talking to believers, concerned about their lack of growth, then tells them of some unbelievers who were close to being saved, but not quite and the consequences for those people, then to hop right back and say that they are confident that they won't end up like that. Well of course they won't. They're believers. How can they end up with consequences directed towards unbelievers? And IMO the warning serves no purpose if it is directed toward unbelievers. The letter was written to believers. And the 5 warnings IMO were also directed toward those same believers. But there was no threat of a possible loss of salvation. There is the possibility of great loss... but not of their eternal life.
That's how I view it, JD.
Let's take a brief (

) look at the context:
There are 5 participal phrases in vss. 4-5 which are difficult to interpret any other way. Isn't it clear that only believers can be said...
- to have been enlightened,
- to have tasted the heavenly gift,
- to have become companions with the Holy Spirit,
- to have tasted the good Word of God, and
- to have tasted the powers of the age to come.
This is the 3rd of 5 major warning sections in the book. What is the warning regarding? Is it warning believers that if they fall away can lose their salvation, or that they will then be severely disciplined by the Lord? Or is it referring to unbelievers? As I see it the warning
is directed to believers - a very severe warning indeed... Now if someone says that these believers can lose their salvation, then logically there's no way out of their saying that once lost, always lost (OLAL). That's a strong argument against the possibility of this referring to a loss of salvation, IMO, regardless of how you view this.
Let me look just briefly at three of those participial phrases:
τοὺς ἅπαξ φωτισθέντας
TOUS APAX FOTISTHENTAS -> "those having been enlightened"
[snipped]
γευσαμένους τε τῆς δωρεᾶς τῆς ἐπουρανίου
GEUSAMENOUS TE TAS DOREAS TAS EPOURANIOU -> "both/and having tasted the heavenly gift"
Te = "and" or"both" This is another aorist participle, dealing with what they tasted or experienced. Also, one should note that Jesus "tastes death" (same Greek word) in Hebrews 2:9, and we would not want to argue that this means He only sampled death but did not really die. The heavenly gift can be seen as a number of different things, but the sacramental understanding is the least supportable and also quite anachronistic. I would argue that this refers to the temporal side of salvation (as opposed to the eternal side, which is the finalization at the judgment).
καὶ μετόχους γενηθέντας πνεύματος ἁγίου
KAI METOCOUS GENATHENTAS PNEUMATOS AGIOU -> "and having become partakers of the Holy Spirit"[snipped] This is again an aorist passive participle, thus showing the outside agency of this action, that they are
made to be partakers, rather than making the move themselves. [snipped]