Who is in view in verses 4-6? A person who is:
- enlightened.
- tasted the heavenly gift.
- partakers of the Holy Spirit.
- tasted the good word of God and the powers of the age to come.
What sort of enlightenment, tasting, and partaking would it be reasonable to think a person has who "falls away" (from what isn't stipulated explicitly)? Is it unreasonable to think such a person was never actually saved? If the "falling away" entailed departing entirely from the faith, John says the following:
1 John 2:19
19 They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us.
Here, then, is scriptural cause to think that those who were enlightened, had tasted and partaken were, despite these things, never truly born again. Can one be enlightened and not saved? Yes. I know atheists, agnostics and Muslims who have a far better handle on the Gospel and the basic doctrines of the Christian faith than the average Christian in North America. They can articulate very clearly the truths of the Gospel, from memory giving chapter and verse, even. I've had discussions with them a number of times over the years and watched a few of them in debates with Christian apologists. To the degree they understand the Gospel and Christian doctrine, they are enlightened to it, but the word does not profit them spiritually, not being mixed with faith (He. 4:2).
Could this not be the same sort of "enlightened" person Hebrews 6:4 describes? I don't see why not. In the NT, every group of Early Church believers that had to be instructed and corrected by Paul or John, Peter or James, demonstrate that its possible to be only partially enlightened to God's Truth. But if genuine believers needed further enlightenment, as the NT demonstrates they did, it's quite reasonable to think that there were non-believers in the Church who also had only a second-hand, fractured understanding of God's Truth, who were enlightened only to a degree, too. Must one think, then, that Hebrews 6:4 is speaking only of a saved person? Not at all. It seems much more plausible to me in light of their "falling away" to understand that the "enlightened" people described in the verse were not truly saved.
What about "partaking"? I've known men and women in the Church who had taught Sunday School classes, served as deacons, led Bible studies and so on who realized they were not actually saved. They had been partakers in the life and work of the Church for decades, in some cases, sharing in what the Holy Spirit was doing in and through their particular congregation of Christians, but were not themselves truly born-again people. It's possible, then, for a person to be a "sharer" in, or partaker in, the Holy Spirit in the second-hand manner of these false converts, praying to God, singing songs of worship to Him, teaching His Truth to others, but not actually truly indwelt by the Spirit.
This sort of "partaking" is suggested, it seems to me, very clearly in the description of having only "tasted" of the things of God (heavenly gift, word of God). If I taste a piece of apple pie, I haven't necessarily eaten the whole thing. In fact, if I say I only tasted something, commonly folk would think I'd had only a sampling of it. Certainly, this could be the case for those "partaking" of the Spirit who have only "tasted of the heavenly gift" (who is the Spirit of Christ). And inasmuch as these "partakers" and "tasters" "fall away," they appear to me to confirm that they had only a second-hand, partial knowledge and experience of the Holy Spirit, and God's Truth.
And then, there's that "if" at the beginning of verse 4. Everything that follows this word is conditioned upon it. In other words, it's impossible to renew to repentance those described in verse 4 IF they were to "fall away." But this means that verse 4 is describing a hypothetical situation, not an actual one. Verse 4 doesn't say that there were such people; it doesn't say that there had been such people; it doesn't say that there would be such people; no, verse 4 describes only a theoretical/hypothetical circumstance that is not concerned with a person losing their salvation but with why a person who had "fallen away" from their change of mind (repentance) concerning Christ and the Gospel couldn't be "renewed again to repentance" about these things.
The enlightened, partaking, and tasting person would have to have had some measure of repentance - a change of mind - about the Gospel and Jesus Christ. If having had this change of mind (to whatever degree), they "fall away" from it, doubting the Gospel and salvation through Jesus, having done so, they become "inoculated" to these things, unable to take them up again. This is all the writer of Hebrews is remarking on in Hebrews 6:4-6, offering a hypothetical, a sort of thought-experiment, not an actual circumstance.
And so it goes, each of your offered proof-texts failing to make the case for your saved-and-lost/works salvation doctrine. It's fascinating to me, though, how you just assume they say what you think they say. Amazing (and not a little disturbing).