It seems that Hebrews 6:4-6 is every so often flung about as a curse by those who want to worry about committing some "unforgivable sin" of falling away to the point that they can never be brought back.
First of all, if you are believing that verse, quoting that verse, and being concerned about it, then this is the mark of a believer. It is the unbeliever, the apostate, the unrepentant, who does not care, and and is not concerned about these things. The unbeliever will mock such things. I remember one time a self-proclaimed apostate bragged in an online Christian forum of being one (a "former" born-again Christian), just to spite us. While eager believers in that forum tried to talk him back into the faith, he just mocked them and went so far as to cite Hebrews 6:4-6 to taunt them, using it as "proof" that he was an apostate and that they were wasting their time trying to bring him back into the fold.
Okay, this is just from reading the English translations. The scriptures are consistent with the message of salvation from the wrath to come, which is conditional only on one point: That we believe that Jesus paid for our sins, justifying us in the sight of God.
If you are content with that and don't want to dig deeper into the original text like I did, then you need not read any further.
But I dug a little deeper and found that this particular verse isn't really talking anything about that to begin with.
First, read the passage in context, from Heb 5:11, up to Heb 6:12. He is telling believers to leave the elementary teachings, and that in the context of him explaining about Melchizedek, etc. (up to Heb 5:10, and then continuing from Heb 6:13) He is only ever talking about us. Read the whole thing.
Of particular interest is a word consistently mistranslated as "laying," as in "not again laying the foundation of repentance ("repentance" = change of mind/thinking) from dead works and of faith upon God" in Heb 6:1. The word is κατα-βαλλω (Strong's G2598, "kata-ballo"), which means "cast down," based on the root word βαλλω (Strong's G906, "ballo") which means to cast or throw with force or violence, the same root word used in εκ-βαλλω (Strong's G1544, "ek-ballo"), which is to "cast out," εκβαλλω used repeatedly with regard to casting out demons from people. In Rev. 12:10, the same word κατα-βαλλω is used, where it says that the "accuser of the brethren is cast down" (not "laid down"). So the issue in Heb 6:1 is that we should not "again cast down the foundation of repentance ("repentance" = change of mind/thinking) from dead works and of faith upon God." Out of context, it may seem that this even contradicts the point I am making, but again, you have to examine it in context.
Here is the teaching, and the question posed: What use is it to keep "casting down" the foundations of repentance ("repentance" = change of mind/thinking) and teaching elementary truths again and again, as if we needed to change our minds again to believe the gospel, the good news? We need to leave the elementary teachings and go on to maturity. The point of the matter is that it is we who cannot be "renewed again into repentance (change of mind), crucifying again for ourselves the Son of God...", since we already are believers, and have already done that. See the different sense I am getting at?
Heb 6:4-6 has a long string of verb participles (and one verb infinitive) in the original Koine Greek discourse, that all follow a single adjective "unable," which prefaces the whole string of participial phrases. Well, that's just Koine Greek for you, which tends to use way more verb participles than English does, going on and on with no punctuation, so it is somewhat awkward and tedious to literally render in English. But read it for what it literally says:
Yes, that is quite awkward to render in English.
The adjective "unable" negates the whole logical proposition that is tied together with conjunctions (for + also + and + and + and + and). It is not grammatically written as a sequence of events, one after another (such as if this happens, then this will happen, then consequently that will happen, then you end up with some final thing happening).
Given that, then there is only one class of people being talked about, and that is his audience, who in verse 9 he says are believers.
In other words, what this is getting at is that since we are unable to be converted more than once, converted people should not keep teaching each other about how to be converted, repeating the elementary gospel to each other over and over, since they do not need to be converted over and over. We only once crucify anew to ourselves the son of God once when we are born again. We are crucified with him, buried with him, and are raised with him. That happens only once, and we are baptized in water only once to show it. Why keep "teaching" each other about it, and "making a spectacle" of it, since we can do it only once? Move on past that teaching!
The "land drinking the rain" (v. 7) is us, bringing forth vegetation. If we bring forth "thorns and thistles" (v.8) then that is not good, because "thorns and thistles" will be burned (note that "land" doesn't burn; the worthless stuff growing on it is what burns). The meat of getting past the "elementary teachings" is producing fruit, not thorns and thistles. This is not about fearing some supposed irreversible curse.
The teaching about Melchizedek is a mature teaching. He digresses to talk about leaving the elementary teachings. It does not make any sense that he would then digress into making a statement about falling totally away from the faith, or cursing apostates, when he is simply urging believers to progress beyond the elementary teachings into maturity.
Now, please do not misunderstand. I am not leading into some Calvinist or hyper-grace teaching to promote some pet "Once-Saved-Always-Saved" doctrine, or whatever. That is not my point. My point is that this particular passage does not talk about "falling away from the faith and becoming an apostate." It talks about leaving the elementary teachings and growing in maturity.
I grant this work to the public domain.