Willful sin is first mentioned in Hebrews.10:26.

26For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, 27But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries. 28He that despised Moses' law died without mercy under two or three witnesses: 29Of 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? 30For we know him that hath said, Vengeance belongeth unto me, I will recompense, saith the Lord. And again, The Lord shall judge his people. 31It is a fearful thing to fall into the hands of the living God.

As soon as we read the above verses, many of us starts trembling. Fear fills all our body from head to toe and people shivers thinking God will not forgive them anymore because they may have committed that willful sin. They think that they have committed the sin so fondly and passionately, even though they are Christians, baptized and knows that that is a huge sin. They think that, even when their conscious is reminding them of God, and reminding them that they should quit the sin, they committed the sin. And also, they did it repeatedly for 5 years or so. Yes, committing a sin intentionally is definitely a huge sin but not a sin that could cause a person to lose his or her salvation. There's nothing to fear if you think your soul is doomed to hell eternally. No one can commit such a bitter sin, that they could lose their salvation. Because the word 'willful sin' is not a word by itself. it is a definition, which we cannot commit. You may ask, how do you know that thing? How will we know anything? We have to go through the Scriptures and check if there is any person or persons who has committed this type of sin? And then we could be able to determine the sin. Let us initiate checking the persons who are driven away from God's presence. First person that has been banished from the salvation is Cain. Cain is the first-born son and first-born human being of Adam and Eve, the first couple on this planet Earth. In Genesis.4:14, Cain says, 'Thou have driven me out this day from the face of the Earth, and from thy face shall i be hid.' So, Cain is the first one in the entire Holy Bible to be casted away from the presence of God. But, if we go through the story of the king David, despite of the infamous 'big sin' which is adultery and the murder of her husband, David still was forgiven by God. See, how God is showing partiality towards his saints. Cain was driven away because he killed his brother, but David was forgiven though he committed adultery and killed her husband. No, that's not true. God shows no partiality. in acts.10:34, it is written "In truth I perceive that God shows no partiality." And Romans.2:11 also says "For God does not show favoritism". David was forgiven but Cain wasn't because there's a reason. Cain actually committed the willful sin. But David wasn't. And there are few more examples in the New Testament also. In Acts.5:1-10, those couple named Ananias and Sapphira also committed the willful sin and lost their souls to hell. What did they do? Just keeping some of their own money aside. Thats it. They both fell down and dead and went straight into the hell because they didn't repent of their sin before they die. So, there's no way they will escape the hell. But why. Cain lost his salvation. Ananias and Sapphira lost their salvation. But David didn't. Why is that? There are many things we have to verify to know if a person had actually committed a willful sin. For me, im 100% sure you didn't ever commit such a sin. As we read previously, Willful sin is not merely a word. It's a definition. We will understand the definition only after we completely learn the subject. This is the first thing we have to learn. Now we will go through the sins they have committed, of the persons who were described before. Let's evaluate their sins, so that we could come out of that fear. But even before that, we have to divide all the reference of Hebrews.10:26-30 into many parts. And we could fully understand what an unforgivable sin is. Those people's account comes up here repeatedly in the entire of our learning. Please, keep on reading until we finish the subject.