My Understanding of Hebrews 6:4-8

Hebrews 6:4 For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, 5 and have tasted the goodness of the word of God and the powers of the age to come, 6 and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt. 7 For land that has drunk the rain that often falls on it, and produces a crop useful to those for whose sake it is cultivated, receives a blessing from God. 8 But if it bears thorns and thistles, it is worthless and near to being cursed, and its end is to be burned. (ESV)

First off, I found this in a commentary:
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The “heavenly gift” is a euphemism for salvation, which the readers have “tasted.” The Greek word for “tasted” is the same here and in v. 5 and is used metaphorically indicating “to eat or drink,” thus experiencing something fully, not merely a superficial participation in something. There is no connotation in the word itself of tasting but not swallowing. This can be seen from the usage in Heb 2:9 [same Greek word in 2:9 as here] where Jesus “tasted” death for everyone, meaning he experienced the full force of physical death. This metaphorical rather than literal usage also precludes a reference here to the Lord’s Supper, although Ellingworth thinks the meaning cannot be entirely excluded. The third participial phrase describes those “who have shared in the Holy Spirit.” The NIV translation renders the noun metochous, “partakers, sharers” with the verbal idea “shared.” The author used this word previously in 3:1 and 3:14 to describe the close relationship that his readers share in the heavenly calling and in Christ. To become a “partaker” of the Holy Spirit indicates primarily “participation in” and denotes a close association with the Holy Spirit, implying reception of the Holy Spirit into one’s life. It is very unlikely the phrase refers to the reception of spiritual gifts of the Holy Spirit, as Hughes suggested.

Allen, David L. Hebrews. Nashville, TN: B & H Publishing Group, 2010. Print. The New American Commentary.
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This means that this passage ONLY applies to someone who has FULLY EXPERIENCED the salvation that comes through Christ.  That this experience is complete and total, someone who does not lack in any way any bit of understanding.  Someone who has "tasted" the Goodness of God in the same way that Christ "tasted" death... not a passing association with it, but a complete understanding.  And who among us has such an understanding?

Secondly, there is the Holy Spirit.  That is, it is the job of the Holy Spirit to call you to repentance, to bring you back into relationship with the Father through the Son.  If you feel, in any way, a desire to be restored to the Father through the Son... that is, if ANYTHING at all inside you makes you desire to pray, or desire to repent, or desire to be in relationship with Christ, then it is the Holy Spirit which is fueling that desire.  Since the Holy Spirit does not act in vain... that is.... if it were impossible for you to come to repentance, the Holy Spirit would not waste His time with you... if you have ANY desire at all within you to be in relationship with Christ, then this verse does not apply to you.    Again, if it were impossible for you to be restored to salvation, the Holy Spirit would not be calling you to repentance. 

What is the mission of the Holy Spirit?  The Spirit's job is to comfort, to console, and to woo.  The Spirit brings us into relationship with Christ, and sustains us in that relationship.  That's why the only unforgivable sin is not blasphemy against the SON or the FATHER, but against the SPIRIT.  What is Blasphemy against the Spirit?  It is to go your entire life and reject the calling of the Spirit.  To go your whole life without responding to the Spirit's call is the highest form of blasphemy.  Thus, if the Spirit is calling you, you have NOT committed the unforgivable sin, and you CAN be restored to The Father through The Son. 

I hope that helps.