If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left,  but only a fearful expectation of judgment and of raging fire that will consume the enemies of God.  Anyone who rejected the law of Moses died without mercy on the testimony of two or three witnesses.  How much more severely do you think a man deserves to be punished who has trampled the Son of God under foot, who has treated as an unholy thing the blood of the covenant that sanctified him, and who has insulted the Spirit of grace?  For we know him who said, “It is mine to avenge; I will repay,” and again, “The Lord will judge his people.”  It is a dreadful thing to fall into the hands of the living God.
In these verses we notice the writer includes himself ("we") as needing this warning. The greek word is translated “we” by definition includes the person writing this word and the people he’s addressing and identifying with. The writer and those he is addressing are believers. This is evidenced by the syntax and specifically the word he uses in regards to the depth of their knowledge of Jesus. It is those who have received a full knowledge (epignosis) of the truth. It is directed to those who deliberately keep on sinning after they fully understand the way of escape in Jesus. It's not head knowledge verses heart knowledge. This idea is foreign to the NT. It's a fictional philophoical theology to prop the doctrine of once saved always saved. It’s not about sins of weakness that come and go in a believers life but a sinful lifestyle that is not yielded to the Holy Spirit. The specific lifestyle of having an idolatrous mind set. For us today we don't necessarily look to Moses and Jesus for salvation or maintaining a righteous relationship with God. The sin of idolatry in this day and age would be trying to be a friend with the world and God at the same time. James 4:4 (NIV)
 You adulterous people, don't you know that friendship with the world is hatred toward God? Anyone who chooses to be a friend of the world becomes an enemy of God. Ineviatably what happens when we try to be friends with the world is that Jesus become recreated in our image. It adds seriousness to the exhortation of verse 25 not to abandon meeting together with other Christians (as the initial Greek gar, "for," indicates).
This recalls John's warning in 1Jn 2:19 concerning those who "went out from us." "Their going," he says, "showed that none of them belonged to us." They had known the way of life, but had not chosen to take advantage of it, and one early sign of heart apostasy is an unwillingness to continue association with true believers. We see this again with Peter when he writes, “2 Peter 2:20 (NIV) If they have escaped the corruption of the world by knowing our Lord and Savior Jesus Christ and are again entangled in it and overcome, they are worse off at the end than they were at the beginning.
Yet despite the advantage of full enlightenment, if there is no change in behavior and sin continues to dominate the life of professed believers, they will find no other hiding place from God's wrath, for there is no other sacrifice than Christ's which will deal for sin. Since by unchanged behavior such individuals give evidence that Christ's sacrifice is rejected, the one way of escape is rejected also. Only judgment and "blazing fire" after death awaits, as one of the enemies of God (2Th 1:7). This behavior parallels those "having fallen away" of 6:6, where apostasy also led to irremediable judgment.
There is a movement in evangelical Christianity in the USA that doesn’t want to acknowledge that God still judges with sternness and finality. They paint a picture that this is an angry God, which they deny exists. All that they are doing is creating a God in their own imagination. This is not the God we see in scripture. If they refuse to paint a correct picture of God, they create a idol and then worship this idol. Then your doctrines are born in the image of this idol you've created. This results in presenting a lie which is not the truth.
The NIV has properly translated the opening phrase of verse 26 as, if we deliberately keep on sinning. It is not a sin one can stumble into suddenly. It is not the normal falterings of a Christian still learning how to walk in the Spirit. It has been well termed "the cancer of non- commitment." It is choosing to live for self behind a Christian veneer and refusing to be delivered from sin's reign by the past sacrifice and present high priestly ministry of Jesus. It is not continual sinning from ignorance as many church members manifest, but occurs after full enlightenment. Such people know of the power of Christ to deliver, but have not chosen to take advantage of it. Their life may appear to be fairly respectable when judged by the world's standards, but what it is like in God's eyes is described in verses 28-30.
The argument proceeds from the less to the greater, very much as the writer had done in 2:2-3. If immediate death was the penalty for violating the law of Moses (which was but a shadow or picture), how much more should one expect severe judgment for continually rejecting, knowingly and deliberately, the reality which is Jesus and his sacrifice! What they have done is threefold:
1. They have trampled the Son of God under foot! The writer chooses a title for Jesus which emphasizes his right to be Lord over all. To trample him under foot is to spurn his right to govern life. Lip service is paid to Christian truth but life is lived as one pleases, even adopting the world's values and standards. As one poet has described it:
He lived for himself, and himself alone;
For himself, and none beside.
Just as if Jesus had never lived,
And as if he had never died!
2. They have treated as something common or trivial the blood of the covenant which has power to make one holy. They have regarded the blood of Jesus as having no more value than the blood of any other man, and therefore, in practice, insisted that religious activities ought to be enough to satisfy God. And they are saying this even though they have previously acknowledged that the death of Christ has ruled out such means. Once they regarded themselves as holy (sanctified) by the blood of Jesus, but now they deny this and reject the cross as unnecessary for acceptance before God.
3. They insult the Spirit of grace. Grace becomes cheap to them. The full understanding of redemptive truth, the awareness that the blood of Jesus can make one holy, the pleasures of meeting together with other Christians; all have been a gracious ministry of the Holy Spirit to the individuals considered here. Now these are being rejected and treated with contempt. It is an egregious insult to the One who was sent to draw men and women to salvation. It actually means to become guilty of the sin where Jesus says in Matthew 7:22-23 (NIV)
 Many will say to me on that day, ‘Lord, Lord, did we not prophesy in your name, and in your name drive out demons and perform many miracles?'  Then I will tell them plainly, ‘I never knew you. Away from me, you evildoers!'