Hebrews 10:26-27

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. 

Willful sinning basically shows a complete disregard for GOD's way of life. Some wrongly believe that there are different levels of sin and that GOD categorizes them as pardonable and unpardonable. For me, its only natural to believe that committing murder is a far worse sin than stealing a cookie out of the cookie jar. I think we confuse the meaning of sin with the meaning of crime whereas crime has degrees of severity but sin is sin.

If we take another person's life in a car accident, for instance, it is a sin to take the life but it was unintentional. , therefore it is forgiveable. If we willfully take a life, then its more difficult for forgiveness. The key is to truly repent, confess and feel the sorrow of your sin deep in your heart. GOD knows whats in our hearts and if your repentance is sincere, I believe GOD will forgive. Through true repentance, repeating the sin shouldn't happen. If willful sin is repeated, then I have to think it is unpardonable and repentance was not sincere. The bottom line is, we should try our best not to sin but when we do, sincerely ask for forgiveness, confess and repent and GOD will keep HIS promise of forgiveness through grace in ourt Lord, Jesus Christ.