Follower, I think you did a great job in presenting why (I believe) the warnings are for actual believers and not just folks who have heard, but rejected the gospel. I want to expand on your work here a bit, in two ways.
The first way is anecdotal. Now, anecdotal evidence isn't usually the best evidence to give in Bible Study, because people's experiences don't necessarily confirm or deny biblical truth. However, the writer of this passage uses this term:
For in the case of those who have once been enlightened and have tasted of the heavenly gift and have been made partakers of the Holy Spirit, and have tasted the good word of God and the powers of the age to come, and then have fallen away, it is impossible to renew them again in repentence.
This term "impossible to renew them again" is pretty key. If the writer has it correct, we should not have even one testimony of someone who 'played church' for years and years, only to out and out reject the gospel, and then, after the rejection, recieve the gospel and show the evidence of a changed heart by showing forth fruit. There cannot be even one such testimony.
However, I've personally known several who have such testimonies. These testimonies are by folks who were basically raised in church, 'went through the motions' so to speak, then (usually in their teen years or early adulthood) go on to totally reject Christianity, only to return and repent and show the truth of their repentence by growing in grace and giving God the glory for it.
In other words, the senario that the writer describes, that of the impossiblity of faux believers, of tares, if you will, ultimately coming to repentance, isn't truthful. I can name names. One such person is a person I know who was actually the head pastor of the church he grew up in, when he realized that he wasn't even a Christian, and that he had been 'playing church' all this time, while inwardly rejecting the gospel.
However, anecdotal evidence is always suspect, I'll admit. Maybe even my preacher friend still isn't truly saved, or maybe he hadn't truly rejected the Spirit before, even though he testifies fully that he had.
So, better still to let Scripture interpret Scripture, and see if the idea of someone who has recieved and believed the word of God can fall away is taught elsewhere in the Word.
Follower has, I think, gave a very credible comparison of the warnings in both Hebrews 6 and 10. Comparing the language of the two chapters we see that this dire warning is to those who:
* have been enlightened
* have tasted of the heavenly gift
* have been made partakers of the Holy Spirit (and how is an unbeliever ever a partaker of the Holy Spirit!?!)
* have tasted the good word
* have tasted the powers of the age to come
* have recieved the knowledge of the truth
Of of the qualities that descibe the person to whom this warning is written, the most compelling are the fact that it is to those who have both recieved the knowledge of the truth and have become partakers of the Holy Spirit. For years I was a member of a Calvinist church who comes down strong on the side of there being no way a believer can ever 'fall away', and never once did I ever hear a convincing arguement as to how someone can be a reciever of the word and a partaker of the Holy Spirit, and yet not be a Christian.
Let's also compare what Christ said again here, because I think it's an important point and I don't want it to get lost. Our Lord then taught that the meaning of His parable about the seeds was this:
And those on the rocky soil are those who, when they hear, receive the word with joy; and these have no firm root; they believe for a while, and in time of temptation fall away. (Luke 8:13)
Now, this is the seed that did grow. According to Matthew, Mark and Luke, a plant did spring from this seed. But, the plant died, it withered. And in the explanation to the disciples regarding this 'seed' it represents someone who does recieve the word, and receieve it with joy, they do believe, they are believers. But, their belief isn't firmly rooted, and when trials come, they 'fall away'.
There is also the writings of Paul, who wrote also to those who faced the same temptation that the recipients of the epistle to the Hebrews faced, namely to return to the Law of Moses for salvation. Paul told those who followed through on that temptation and turned back to circumcision that "You have been severed from Christ, you who are seeking to be justified by law; you have fallen from grace."(Galatians 5:4)
Now, the key phrase here is that they have been 'severed from Christ'. How can one be severed from someone, if one has not been bound to that person?
I could write more, but it's late, and I think I'll get back to bed now. (My little boy woke me up at 3:00 am having an asthma attack. He's fine now, and sound asleep, but I've been wide awake. :o )
I think there is enough to ponder over here though. We have the writer of Hebrews, Paul and even Jesus, all in agreement, teaching that some who recieve, believe, become partakers of the Holy Spirit, have evidence of a new life within them (the seed did grow), are bound to Christ, can yet still fall away.