I agree, Hebrews 6 definitely deserves close attention. If the author of Hebrews is indeed propounding that eternal salvation can be lost, the situation is even more serious than many realize.
Hebrews 6:4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
5 And have tasted the good word of God, and the powers of the world to come,
6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
I have met many Christians who believe that eternal salvation can be lost; however, I have met very few that believe that salvation, once lost, cannot be regained again. However, (if the author is propounding that eternal salvation can be lost) this is exactly what is asserted in Hebrews, "It is impossible....to renew them again unto repentance".
Following this line, it can be deduced that a believer, take King David, for instance, is sizziling in Hell as we speak. He denied his Lord by his evil works I.e. murdering Uriah and stealing his wife -- and not only this, he lived in a state of unrepented sin for almost a whole year. How can this be, when it is taken into account that David is called, later in the Bible, a "man after God's own heart"? A man after God's own heart that is damned? The thought (for me) is almost inconceivable.
Of course, I don't believe that the previous interpretation of Hebrews 6:4-6 is the correct one. The author of Hebrews is not speaking of a person who has lost their eternal salvation; simply of one who has lost the ability to repent. This is corroborated elsewhere in Scripture. Paul spoke of some believers whose sin was so grave that the church should:
...deliver such an one unto Satan for the destruction of the flesh, that the spirit may be saved in the day of the Lord Jesus.
1 Corinthians 5:5
Note that he says "that the spirit may be saved in the day of the Lord Jesus"; the person lost his/her life, not eternal salvation.