"Jesus our High priest has taken away sins for everyone.. Where there is forgiveness, there is no longer any offering for sin. We can be confident to enter the holy place by the blood of Jesus, by a new and living way which He inaugurated for us through the veil, that is, His flesh, and since we have a great priest over the house of God, let us draw near with a sincere heart in full assurance of trust, having our hearts sprinkled clean from an evil conscience and our bodies washed with pure water (this speaks of being saved & reborn by trusting Christ)..
If we go on sinning willfully after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, but a terrifying expectation of judgment and the fury of a fire which will consume the adversaries. Anyone who has set aside the Law of Moses dies without mercy on the testimony of two or three witnesses. How much severer punishment do you think he will deserve who has trampled under foot the Son of God, and has regarded as unclean the blood of the covenant by which he was sanctified, and has insulted the Spirit of grace? For we know Him who said, “Vengeance is Mine, I will repay.” And again, “The Lord will judge His people.” It is a terrifying thing to fall into the hands of the living God.. Do not throw away your confidence (in God as a Jew), which has a great reward. For you have need of endurance, so that when you have done the will of God (trusted in Christ, gotten saved), you may receive what was promised." (Hebrews 10:11, 18-22, 25-36)
"If we go on sinning willfully after receiving the knowledge of the truth.." The only way to sin willfully is to reject Christ because Christ makes it impossible for someone to sin willfully through rebirth. Rebirth concerns the birth of a new identity as a perfect spirit which "is not able to sin" (1 John 3:9). "The knowledge of the truth" is the good news of what Jesus Christ has done which is described for the Jewish audience here in the letter to the Hebrews.
"Where there is forgiveness, there is no longer any offering for sin.." "There no longer remains a sacrifice for sins.." Forgiveness has already been given out through the sacrifice of Christ thus no other offering for sin exists, He was the last one & He got the job done. He is the last offering & last sacrifice, literally. The focal point isn't a warning about your sin points racking up because you've rejected Christ. It is focused on the all sufficient sacrifice of Christ for all mankind. Any offering or sacrifice a Jew would now make is worthless because sin has already been taken care of, the problem is solved and the solution is Jesus!
"Those who have trampled under foot the Son of God, and regarded as unclean the blood of the covenant, and insulted the Spirit of God's love for us (grace) are deserving of immense punishment." The Jews are, for all time, in some way "God's people." So it says, "The Lord will judge his people."
How can a loving God punish these people? Imagine someone you love inviting a group of old friends over for dinner & cooking them an elaborate meal. When they come over they chew up the food and spit it in her face, smash the dishes, & throw her on the ground to stomp on her. It is fully loving to be angry with these people. Your anger is on behalf of the person whom you love. Likewise the Father has anger on behalf of those who hate & invalidate His Son. God did not hold anything against those who physically persecuted & killed Jesus but those who insult His love, those who stomp on Christ as if He were worthless are asking to "fall into the hands of the living God" for punishment. They have decided to be His adversaries.
Note a uniquely Jewish statement here, they "regarded His blood to be unclean" thus His covenant to be unbinding. The idea of unclean blood & covenants only pertain to Jews, not Gentiles of the day. The Jews alive during the resurrection of Christ who have the chance to outright reject Him are the unique group of people described here. Perhaps there are no other people in history whom this applies to. Interestingly God says to forgive those who do bad things to you. To my knowledge He doesn't say to forgive those who hurt the ones you love. This portion of Hebrews 10 is written as a warning but more so as an invitation to trust in the Son of God & live. The writer speaks of Christ taking care of sin & making a new covenant, then of how to be saved, & finally what happens if they rejected Christ after hearing all this truth.
None of this is aimed at a reborn child of God. These are not warnings about losing your salvation. The idea that one could lose their salvation is like a fireman who saves you from a burning building later deciding to murder you because you did or said something he didn't like. "Whoever trusts in the Father has eternal life and will never be condemned; that person has crossed over from death to life" (John 5:24). How can God extinguish eternal life?
Hebrews makes the case for Jesus as Christ & reveals the impact of His sacrifice to the Jews who were alive during the time of Jesus' resurrection (saved or unsaved). It is sad that the mere existence of any statement in scripture makes Christians think that it applies to them. If, as a Christian, you see a scripture that lessens your peace, causes you fear, makes you feel less loved by God, or lessens your confidence in God & His goodness it is either not aimed at you or you have already believed various falsehoods that make you misinterpret it. The Spirit of Jesus brings peace & drives out fear. He causes you to become more confident in Him and less confident in falsehood. If you know you have trusted Jesus and any scripture makes you fearful (less peaceful) you have misapplied it or misinterpreted it. Reconsider the meaning or just pay no mind to it, letting God teach you the meaning if He so chooses.