Continued from Saturday
... However, the strongest warning by far in Hebrews comes in 5:11-6:8. After the author has started to discuss Jesus as a priest, he suddenly interrupts his train of thought to give the sternest warning to the audience in the whole sermon. They have been believers long enough to be teachers but they are back on milk again, presumably because they don't understand that Christ has now made the sacrifices of the Old Testament unnecessary.  The author warns them in the strongest of terms that if they don't watch out, they may end up in the fires of God's judgment, even though they started out so well.
First, the sermon slaps them for being babies. They can't tell the good from the bad, the author scolds them (5:14). It's like they need to go back to elementary school and learn their ABCs all over again (5:12). They can't see the clear path of righteousness.
The author shames them. When they first believed, they had to learn about not sinning and about trusting in God (6:1). They learned about Christian baptism and about laying hands on people, perhaps to receive the baptism of the Spirit (6:2). When they first believed, they learned about the resurrection and eternal judgment for those who are not going to be saved. 
They need to move on from these ABCs. Now, they need to trust in Christ for atonement and let go of their felt need for the temple. They needed to go on to maturity (6:1). Some of this heightened shame tactic is meant to move them to action--the author doesn't really think they will fall away (6:9). But this sort of shock therapy was a culturally appropriate way of scolding them into movement again.
The most shocking statement of all no doubt comes in 6:4-8. They have come so far. They have received the Holy Spirit and thus tasted the glory of heaven that is coming. They have experienced the power of the coming age. If they fall away at this point, Hebrews says, they will never find their way back. It would be impossible for them to get to a place of repentance again.
Hebrews 12:16-17 use Esau as a model of someone who sold his birthright and then tried to find a way back to the blessing. But although Esau tried, "he found no place for repentance, though he sought for it with tears" (12:17, NASB). The parallel for the audience is obvious. If they sell their "birthright," if they toss away their sonship (e.g., 2:11-13), they will not be able to "be brought back to repentance" (6:6). Rather, their corpses will fall in the desert, short of the Promised Land, like the wilderness generation (3:17).
Hebrews says to toss Christ away at this point would be like trying to crucify Jesus all over again (6:6), putting him up on the cross to humiliate him again. Hebrews 10:26 uses the image of using up the sacrifice of Christ and having no more blood left. Obviously this is just a picture but it is truly a shocking one. To turn away from Christ deliberately when they have come as far as the audience, that would be like insulting the Holy Spirit (10:29). It would be like throwing the blood of Jesus on the ground and stomping on it.
The warning in 6:7-8 ends with yet another halting image. If you keep watering a piece of land over and over again, but the only things that grow are thorns and thistles, that land is destined for burning. The author surely made his point clear. Having come so far at such a key point in history, the audience had best think carefully before they turned away from the living God! "It is a dreadful thing to fall into the hands of the living God" (10:31).
 It is the mention of Christ as a priest like Melchizedek that sparks this interruption, reinforcing our sense that it is the final sufficiency of Christ's atonement that is key to their wavering faith in some way.
 It was this list that changed my mind about whether the audience was primarily Jew or Gentile. This is not the sort of list that a Jewish believer in Christ would learn when first believing, but it is the sort of list a new Gentile believer would learn when converting.