"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, 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." NIV translation
Paul quotes from the Law and reminds the people that it is God who will judge those who reject His ways. Jesus is The Way and those who reject His mercy and grace can only expect to be judged by Him. If you want to be judged by the Law of Moses, God will grant you your wish. The same situation is happening even more today as each denomination hangs onto a different part of the law and try to add it to the grace of God. In so doing, many are led into the willful sin that Paul was warning the Jewish believers about.
"Remember those earlier days after you had received the light, when you stood your ground in a great contest in the face of suffering. Sometimes you were publicly exposed to insult and persecution; at other times you stood side by side with those who were so treated. You sympathized with those in prison and joyfully accepted the confiscation of your property, because you knew that you yourselves had better and lasting possessions." NIV translation
Here, Paul reminds the Jewish believers of the joy of grace. When they had first came to faith in Jesus Christ, they had realized that it was only a gift from God and the joy followed because they knew freedom for the first time in their lives. This joy and freedom gave them the courage to stand in the face of opposition as they looked forward with certainty to their rewards from God.
"So do not throw away your confidence; it will be richly rewarded. You need to persevere so that when you have done the will of God, you will receive what he has promised. For in just a little while, 'He who is coming will come and will not delay. But my righteous one will live by faith. And if he shrinks back, I will not be pleased with him.' But we are not of those who shrink back and are destroyed, but of those who believe and are saved." NIV translation
Is Paul talking about losing your salvation? Absolutely not! The throwing away of the confidence speaks of looking back to the law to somehow add to our assurance of salvation. He urges the Jewish believers to hold onto the grace that has been given to them and to resist the temptation of works. Those that "shrink back" never really had the confidence of salvation and so were never really saved. Are you confident in the sufficiency of Christ?