Those whom God has called to receive the knowledge of salvation will not be given a second chance if they sin willfully. Hebrews 10:26-29; 6:4-6. To receive this punishment, though, it was first necessary for them to have known God's truth (Hebrews 10:26).
God will forgive any sin that is repented of—even if, through weakness, temptation, or habit, the sin is repeated. As long as a person's intention, attitude, and desire is to obey God, he will repent, and God will forgive him (I John 1:9). Jesus said we ought to forgive 70 times seven times (Matthew 18:21-22). God is even more willing to forgive than that!
So what is a willful or an unforgivable sin?
If a person has been given spiritual understanding of God's way and has experienced the blessings of that way, but decides he no longer wants it, changes his attitude, deliberately chooses a life of rebellion, despising God's laws, then his sin is unforgivable because he will not repent of it! God will not give eternal life to anyone who insists on living Satan's way.
Such people have been specially called by God and given His Spirit, yet at some point in time they purposely, knowingly rejected it! Perhaps through bitterness, refusal to forgive others, or neglect of prayer and Bible study, they stopped allowing God to build His character in them.
Like the servant who did nothing with his talent (Matthew 25:24-30), some failed in God's purpose for them. They became so calloused to their negligence and their sins that no matter what God could do to try to correct them, they will never want to repent.
God gives everyone all the help, encouragement, and opportunity for success they need to inherit eternal life. God will never quit helping anyone! But these people have quit on God! They show by their actions that they do not really want eternal life in God's Family—and so God, in love, will not give it to them.
To guarantee peace, prosperity, happiness, and success throughout all eternity, God cannot allow these selfish people to make life miserable for others around them. Although God certainly has no pleasure in the death of the wicked (Ezekiel 18:31-32), these sinful people must be put out of the way.