Regarding Hebrews 6:4 – 6 Here are some textual points with interpretive inlays.  My translation below is from the Greek and clarifies, in English, the relationship of the verbs (participles) in this long sentence.
1) those participating in the life of God (v.4 – 5) can fall away
2) whatever “restore them again to repentance” means–personally I like the points about jewish culture and the temple practices, specifically returning to blood sacrifices rather than trusting Jesus’ blood sacrifice of Himself, at the time as the historical cultural backdrop here because the book is written to Jews likely in Jerusalem–once a full fledged rejection of the life of repentance based on the crucifixion occurs, there is no way to return to God because this was the only way to get to Him in the first place.  Sure, if we are Jews living in Jerusalem at 65 A.D. then our attempt to  be “restored to repentance” might be the action of bringing a blood sacrifice to the temple. But if we are secular Americans today, our rejection of the faith might look like a return to the party life, perhaps thinking this is the way to “make the most of life.”
3) Additionally on this last point, the question remains whether the person cannot ever return once they fall away; a quick reading of the text might suggest this; however, I think the text is more fully pointing to that while in this “falling away status” they cannot return to repentance.  Take this translation for this point, paying attention especially to the tenses (time) I have in parenthesis next to each bolded verb below:
“For to restore (literally, “to be restoring”: present) to repentance again—those who, to their own eyes, are crucifying (present) again the Son of God and disgracing Him (present)—is impossible in regard to those who once were enlightened (past) and tasted (past) the gifts of the heavenly realm and became (past) sharers of the Holy Spirit and tasted (past) the good word of God and the powers of the age to come.”  
I hope it is clairvoyant that the crucifying and disgracing is happening presently and at the same time as the impossibility of the restoring. So when presently crucifying again and disgracing Jesus they cannot be restored.  There is a time line here; this person is someone who had tasted and was enlightened but presently is disgracing Jesus and so it is impossible for him/her to repent.
“to restore to repentance again”: I would point out here that it does not say “to restore to repentance ever again”
“who once were enlightened”: the word “once” here could equally mean “at one time” or “once for all.”  Either way, there would not have to be a “new enlightenment” for a fallen believer to take up and accept and trust the knowledge formerly imparted by the Holy Spirit.
“to their own eyes”: This shows us that those who abandon the faith after first accepting it are saying that Jesus got what He deserved.  It is not that somehow Jesus is out there (metaphysically) getting re-crucified every time someone loses his or her faith.  The “to their own eyes” means in their attitude or according to their understanding.  This person would have believed in Jesus at some point and learned of Jesus’ unjust crucifixion at the hands of those who did not believe in Jesus and these thought He deserved to die. Think of this person standing with John and Jesus’ mother during the crucifixion, watching in sorrow. But when this person falls away he or she joins the side of the crucifiers, saying effectively, “I once was with those sorry about Jesus’ death but now I do not believe in Him and so, hand me the spear and give me the nails, I’ll pound them in
 and stab Him because He is only getting what he deserves.”
4) Some people use their theology to decide what this passage should say; I am trying to avoid that.  For instance, I generally believe in eternal security (that you cannot lose your salvation) but I am not so arrogant as to silence this text by making it fit my theology.  The word “fall away” literally means “apotasize” or “to commit apostasy” and so, unless we want the author of hebrews describing something that could never happen, but making it seem like it could, we should accept its possibility, even if it does not fit with the rest of our theology.
I would point out that, in day to day interactions, we might readily feel deceived or misled if someone we trust presented something to us as though it were a real possibility when it was not. Imagine this: “If you speed and get caught, you are going to jail.”  Then imagine the anxiety you would have once pulled over only to find out that your “trusted friend” was just making up a worse case scenario to get your attention.  We might credit this “trusted friend” with true care but the trustworthiness of that friend will be in question if their trustworthiness is not already downgraded from this one situation.  Simply, the use of hyperbole will not wholly satisfy the notion that our “trusted friend’s” tactic was on the level.  But, then again, who ever said God was safe or “on the level?”
5) This is one of the most difficult passages in the Bible to interpret so we should not feel too much pressure to have to agree with anyone but, rather, measure and think through the reasons for ourselves.  But do not just pick whatever version you might like best (this is basically to play god with God’s word: dangerous) but weigh the interpretations according to their convincing reasons and arguments.
6) lastly, the book of Hebrews is packed with threats about the possibility of losing one’s salvation (e.g., Heb. 10, 4, etc.) and so the real possibility of it occurring and being described in Heb. 6 would fit the broader context well. I do not find categorizing the whole Book of Hebrews as “sermonic” or “homiletical” as solving the problems these threats pose.  This categorization is designed to explain why all of the threats are just “hypothetical” and so cannot happen.  This is not convincing to me because of the terrible seriousness the threats impose.  And let’s forget the human author of Hebrews for the moment.  So God the Spirit says, “As I swore in my wrath, they [the disobedient] shall not enter my rest” (Hb. 4:3) . . . Let us therefore strive to enter that rest so that no one may fall by the same sort of disobedience (4:11).”  But really, following the “hypothetical threat logic” above I find unconvincing, the Spirit is saying by way of this, “I am only saying this to make sure you make it and do not fall by the way side even though I swore an oath not to let the disobedient enter it.”  So now God can swear and oath and not mean it; this is more trouble them its worth to affirm eternal security because if I have to pick which one is more central to God’s character and is more consistent with the Scriptures, I am going with God’s goodness (and inability to deceive or do evil) over eternal security ever time.  There would be no such possibility for eternal security, after all, if God were not good in the first place.
And for those who might find my citation of Hebrews. 4:3 objectionable because you might think that that text only applies to those from the book of Numbers (14, 20), I would point out that the author of Hebrews is citing that passage in Numbers in the first, then citing Ps 95 (which is citing Num) which holds out the fact that people can still not enter God’s rest due to disobedience, and then the author of Hebrews, frighteningly, applies those OT texts to the church.
B. T. Scalise
Technical Greek stuff here (so ignore it if its a bore!):
1) And for those Greek scholars out there, the difference in the aspect (following aspect theory) between the present and the aorist tense would still indicate a similar conclusion.  The emphasis of the present tense following aspect theory would be on its continuous, and so current, nature: “For to be restoring (continuous/imperfective aspect) to repentance again—those who, to their own eyes, are crucifying again the Son of God and disgracing Him (both continuous/imperfective aspect) . . . .” Even if the objection is raised that the aorist tense itself is only indicating aoristic aspect (or undefined) the author of Hebrews begins his list of verbal ideas describing this person (enlightened, tasted, became, tasted) with hapax which establishes some former time (hapax = at one time, once, once and for all) via this adverb rather than the verbs (ptcs) at all.
2) And for those who might be suspicious of my translation which differs at the beginning from almost all other translations, my transition resists displacing the true referent (subject: anakainizein, to restore) with the ambiguous “it”: “it is impossible . . .” is the typical way 6:4 is rendered but the “it is” is implied.  I, of course, do not disagree that “it is” is a completely legitimate translation.  Implying only the “is” after bringing the infinitive up next to the adjective “impossible” as in my translation (For to restore again to repentance is impossible) follows typical predicate adjective construction.  The Greek, moving the infinitive up would look like this: ανακαινιζειν γαρ παλιν αδυνατον . . . .” By doing this, the nominative element (in this case, anakainizein) is brought to the front and, following my proposed translation above, all of the accusative elements are grouped together