I was looking for this post and it led me to post the following in an older thread. Hopefully it will help someone.
Heb 10:26, For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins,
Heb 10:27, But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries. Heb 10:28, He that despised Moses' law died without mercy under two or three witnesses: Heb 10:29, 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? Heb 10:30, 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. Heb 10:31, It is a fearful thing to fall into the hands of the living God.
We must understand this passage by certain biblical knowledge.
First, we find that according to scripture, Christians are not under the law (Romans 6:14), are dead to the law (Romans 7:4, Galatians 2:19) and are delivered from the law (Romans 7:6). Where no law is (where it does not apply), there is no transgression (Romans 4:15); sin is not imputed where there is no law (where it does not apply) (Romans 5:13).
We find in holy scripture, therefore, that "all things are lawful for me" (1 Corinthians 6:12, 10:23).
Now, sin is the transgression of the law (1 John 3:4); so I conclude that as a believer under grace, I cannot sin in the eyes of the Father (1 John 3:9); who looks down on me from heaven and sees the shed blood of my Lord and Saviour Jesus Christ.
So, looking at the passage, how is it possible to sin willfully after having received the knowledge of the truth? All of the things that I have mentioned above are obtained by faith in Jesus and what He did for us on the Cross.
So, I would sin willfully by failing to place my faith in Jesus and what He did for me on the Cross. In such a situation, the sin of transgressing the law would not be covered and I would be sinning against the Lord.
To sin willfully, I think that it would have to be a situation where I am not tempted to sin and a situation where I do not sin out of weakness. It is written,
Psa 37:23, The steps of a good man are ordered by the LORD: and he delighteth in his way.
Psa 37:24, Though he fall, he shall not be utterly cast down: for the LORD upholdeth him with his hand.
I may fall into some temptation; but if I sin out of weakness then there is the possibility that I have not sinned willfully against the Lord; and I ought to look up in my faith and believe that there is indeed forgiveness for me, even in light of this passage.
If I keep faith in Jesus and what He did for me, then I am blessed according to the following scripture.
Rom 4:7, Saying, Blessed are they whose iniquities are forgiven, and whose sins are covered.
Rom 4:8, Blessed is the man to whom the Lord will not impute sin.
As long as sin is not imputed to me, I cannot sin willfully against the Lord, in the Lord's sight.
So then, the willful sin that is being spoken of in the passage has to do primarily with rejecting faith in Jesus Christ and what He did for us on the Cross.