Answer:

The Passage in Hebrews is talking about those who, once becoming a Christian, choose to return to willful sin over a continued time, making a mockery of the grace of God and the blood of Jesus.  The Hebrew writer says that to do so amounts to trampling the Son of God under foot and insulting the Spirit of grace.  Such a person has "fallen away."  For him or her, what they await is the judgment of God and being sent to hell.  That this is what the Hebrew writer has in mind is supported by Hebrews 6:4-7 who says that it is impossible for those who have been saved (enlightened, shared in the Holy Spirit, etc….) who fall away, to be renewed to repentance.  Hebrews 12:14-17 teaches the same idea, advising that if we are like Esau, we can get to a point at which it is simply not possible to repent and be right with God again.  Truly, if we are saved and choose to insult the Holy Spirit which comes to be in us, causing him to leave the temple he has occupied, it is a dreadful thing, as we will fall into the hands of God and be judged.  This is the "unforgivable sin."  This is "blaspheming the Spirit."  Hebrews 10:26-31 is a truly dramatic warning against taking the grace of God for granted and turning back to a life of sin.

John Oakes