This verse is truly troubling me...
Hebrews 10: 26 For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins,
It used to be very troubling for me too tjw. Until...I took the time to read that whole letter (hebrews) for myself, then when I came to that partial sentence, it did not trouble me at all, in fact, it made me sing for joy.
The sin Paul is speaking of is rejecting the knowledge of the truth, for receiving the knowledge of the truth is what brought us out of sin. The old covenant is what declared us sinners in the first place, the new covenant brought us out of the whole realm of sin . The truth was that the sacrifice of Christ has made you right with God, perfected forever. If you reject that truth, and go back to the law, then what is going to save you? Seeing you are trampling God's own provision for you. You will just be in fearful dread of judgment.
You can spend your time here on this earth, believing Christ's sacrifice is sufficient, and actually look forward to death, because you know that's when you're headed back home. Or, you can spend your time here on this earth, not believing Christ's sacrifice is sufficient, and be fearful of death, because you are not sure of what's going to happen at death, -even though JC told you plainly that what is going to happen is that he is going to receive you unto himself, that where he is, you may be also.
Either way, whether you believe the sacrifice was sufficient or not, doesn't mean it was ineffective. Our belief only affects our perception, not the truth itself. Truth doesn't change, our belief of it or not does.