How would you describe willful sin? Can you explain to me what willful sin is?
Willful sin is a purposeful choice to sin, even though you know God is against it and that you should not do it. It means you are putting your will above God's.
Parents often see this in their own children. There are times they are disobedient because they forgot, or the lure of doing something they knew they shouldn't was just too strong. But there are those times when the child just looks the parent straight in the eye and dares the parent to stop him -- that is willful disobedience. He doesn't care that the parent says it is wrong, he thinks he knows better and that he is in control of the situation.
People sometimes do that with God. "For if we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries" (Hebrews 10:26-27). This is not talking about sins done in ignorance or a person's will to not sin being worn down. This is a person who knows what is right and wrong, who purposefully chooses the wrong, and dares God to stop him.
Korah's rebellion is a good example of willful sin. Korah got it into his head that he should be able to lead the children of Israel. He gathered a bunch of men and "They gathered together against Moses and Aaron, and said to them, "You take too much upon yourselves, for all the congregation is holy, every one of them, and the LORD is among them. Why then do you exalt yourselves above the assembly of the LORD?"" (Numbers 16:3). In Korah's mind, because he was a Levite whom God picked to serve in His tabernacle, he could demand the right to do what Moses had been doing. This wasn't a mistaken idea. Korah had seen the power of God and knew that Moses was God's chosen leader. The argument was a purposeful ploy to wrest control away from Moses. Moses offered a challenge to prove who was in the right, but he also gave warning. "Is it a small thing to you that the God of Israel has separated you from the congregation of Israel, to bring you near to Himself, to do the work of the tabernacle of the LORD, and to stand before the congregation to serve them; and that He has brought you near to Himself, you and all your brethren, the sons of Levi, with you? And are you seeking the priesthood also? Therefore you and all your company are gathered together against the LORD. And what is Aaron that you complain against him?" (Numbers 16:9-11). Moses is accusing them of knowingly and purposely rejecting the choices God made. The acceptance of the challenge shows us men who were willing to do what they knew God disapproved of and basically daring God to do something to stop them.
"And Moses said: "By this you shall know that the LORD has sent me to do all these works, for I have not done them of my own will. If these men die naturally like all men, or if they are visited by the common fate of all men, then the LORD has not sent me. But if the LORD creates a new thing, and the earth opens its mouth and swallows them up with all that belongs to them, and they go down alive into the pit, then you will understand that these men have rejected the LORD." Now it came to pass, as he finished speaking all these words, that the ground split apart under them, and the earth opened its mouth and swallowed them up, with their households and all the men with Korah, with all their goods. So they and all those with them went down alive into the pit; the earth closed over them, and they perished from among the assembly" (Numbers 16:28-33).
What Korah did was called blasphemy. It is the purposeful slandering of another's reputation. "But when you unwittingly fail and do not observe all these commandments, which the LORD has spoken to Moses, even all that the LORD has commanded you through Moses, from the day when the LORD gave commandment and onward throughout your generations, then it shall be, if it is done unintentionally, without the knowledge of the congregation, ... Also if one person sins unintentionally ... But the person who does anything defiantly, whether he is native or an alien, that one is blaspheming the LORD; and that person shall be cut off from among his people. Because he has despised the word of the LORD and has broken His commandment, that person shall be completely cut off; his guilt will be on him'" (Numbers 15:22-24, 27, 30-31).
None of this is to say that a person guilty of willful sin can't repent and return to God. The point God is making is that it doesn't often happen. When a person goes against God with an "I dare you to try stopping me" attitude, that person is so prideful that he isn't likely to humble himself and admit he was wrong. That is why the writer of Hebrews warned Christians that they can't bring such a person back to God. "For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit, and have tasted the good word of God and the powers of the age to come, if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame" (Hebrews 6:4-6). The person has to make his own choice. I and others can't persuade him out of his foolishness because he has purposefully, knowingly, and defiantly left God.
It is probably one of the saddest situations I know of. I've known a number of friends do this. Two young men whom I dearly love did this this year. I hate being helpless. I pray that they will come back, but I know it is basically out of my hands and completely in their own. Paul, after all, was once a blasphemer but turned around (I Timothy 1:13), so there is always hope others will turn as well.