For "aveirot" against God, whether or not God is willing to grant forgiveness depends on the seriousness of the repentance, which depends upon the sinner's willingness to change his behavior in the future. For an "aveira" that is committed intentionally, knowing at the time that the action was an "aveira", achieving forgiveness is hardest. God may be willing to forgive if the sinner was acting rebelliously at the time but has since decided to repent for the rebellious behavior, or if the sinner acted on an uncontrolled impulse, but has since learned to control his behavior.