Hi LostChildinTheMidst,
The verse you're referring to, when understood in context, is not talking about "willful sin" in the sense of giving into temptation or continuing to struggle with a particular sin. Hebrews 10 is all about being saved by faith and the warning about rejecting this gift of salvation, which is found in Christ alone, thereby "trampling the blood of Christ underfoot. I think it would help to take a close look at the entire chapter.
Hebrews 10
1 For the law having a shadow of good things to come, and not the very image of the things, can never with those sacrifices which they offered year by year continually make the comers thereunto perfect.
(here we see that the Law is but a shadow that pointed to Christ, as the sacrificial system was inadequate and could not save anyone)
2 For then would they not have ceased to be offered? because that the worshippers once purged should have had no more conscience of sins.
(If these repeated sacrifices had been sufficient to perfect those making them, then they would not have had to been continually repeated.)
3 But in those sacrifices there is a remembrance again made of sins every year.
4 For it is not possible that the blood of bulls and of goats should take away sins.
(Animal sacrifice cannot save anyone from sin.)
5 Wherefore when he cometh into the world, he saith, Sacrifice and offering thou wouldest not, but a body hast thou prepared me:
6 In burnt offerings and sacrifices for sin thou hast had no pleasure.
7 Then said I, Lo, I come (in the volume of the book it is written of me,) to do thy will, O God.
8 Above when he said, Sacrifice and offering and burnt offerings and offering for sin thou wouldest not, neither hadst pleasure therein; which are offered by the law;
9 Then said he, Lo, I come to do thy will, O God. He taketh away the first, that he may establish the second.
(Jesus came to do the Father's will, that He would be the ultimate sacrifice for sin. He alone takes away the sin of the world as the perfect, spotless Lamb of God. By His sacrifice He declares that animal sacrifice is not sufficient and only He (Christ) can take away sins, forever and completely.)
10 By the which will we are sanctified through the offering of the body of Jesus Christ once for all.
(Christ's [one time for all] sacrifice is entirely sufficient for our sanctification.)
11 And every priest standeth daily ministering and offering oftentimes the same sacrifices, which can never take away sins:
12 But this man, after he had offered one sacrifice for sins for ever, sat down on the right hand of God;
(Unlike the sacrifices the priests offered, which had to been continually repeated)
13 From henceforth expecting till his enemies be made his footstool.
14 For by one offering he hath perfected for ever them that are sanctified.
15 Whereof the Holy Ghost also is a witness to us: for after that he had said before,
16 This is the covenant that I will make with them after those days, saith the Lord, I will put my laws into their hearts, and in their minds will I write them;
(We have a new covenant with God, through Christ and His sacrifice)
17 And their sins and iniquities will I remember no more.
18 Now where remission of these is, there is no more offering for sin.
(There is no other offering but Christ's death on the cross. His sacrifice alone takes away sin.)
19 Having therefore, brethren, boldness to enter into the holiest by the blood of Jesus,
20 By a new and living way, which he hath consecrated for us, through the veil, that is to say, his flesh;
21 And having an high priest over the house of God;
22 Let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience, and our bodies washed with pure water.
(We can and should have full assurance of our salvation, because Jesus' sacrifice is absolutely sufficient and we have Him as our High Priest as well.)
23 Let us hold fast the profession of our faith without wavering; (for he is faithful that promised
24 And let us consider one another to provoke unto love and to good works:
25 Not forsaking the assembling of ourselves together, as the manner of some is; but exhorting one another: and so much the more, as ye see the day approaching.
(We should always love and encourage each other as brothers and sisters in Christ, encouraging in love and good works and also regularly meeting together in fellowship, especially in these last days.)
26 For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins,
27 But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries.
28 He that despised Moses' law died without mercy under two or three witnesses:
29 Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace?
The verse to which you are referring (highlighted in bold and underlined above) makes sense when we keep reading. We can then see that the "willful sin" being discussed is the treading underfoot of the Son of God, despising the "blood of the covenant" that has sanctified us, and considering it an "unholy thing." All that to say that the "willful sin" is rejecting Christ's sacrifice for sin (the only sacrifice that is sufficient) and considering it to be unworthy.
30 For we know him that hath said, Vengeance belongeth unto me, I will recompense, saith the Lord. And again, The Lord shall judge his people.
31 It is a fearful thing to fall into the hands of the living God.
32 But call to remembrance the former days, in which, after ye were illuminated, ye endured a great fight of afflictions;
33 Partly, whilst ye were made a gazingstock both by reproaches and afflictions; and partly, whilst ye became companions of them that were so used.
34 For ye had compassion of me in my bonds, and took joyfully the spoiling of your goods, knowing in yourselves that ye have in heaven a better and an enduring substance.
35 Cast not away therefore your confidence, which hath great recompence of reward.
36 For ye have need of patience, that, after ye have done the will of God, ye might receive the promise.
(And Scripture tells us the will of God is to believe on the Son)
37 For yet a little while, and he that shall come will come, and will not tarry.
38 Now the just shall live by faith: but if any man draw back, my soul shall have no pleasure in him.
(And here it is confirmed again, we live by faith.)
39 But we are not of them who draw back unto perdition; but of them that believe to the saving of the soul.
(And again, we believe on the Son and His one time sacrifice that saves us.)
You may also want to continue reading Hebrews 11. In this chapter you will find what is referred to as "the Hall of Faith" because it talks about many OT people who were justified not by works, but by their faith and trust in God.
I hope this helped, but if I wasn't clear enough, please feel free to ask me to explain.
