The statement, "God forgives all sin" is not Biblical. While God does forgive sin, there's one specific sin, if committed, that he will not forgive. This sin is called the blasphemy of the Holy Spirit. More
Before I even begin, let me say from the outset that because of the very nature of this teaching, there are going to be those who read it and due to their own spirit of condemnation that they walk in, they'll think, "This is me! I'm lost forever!". The devil will play on their minds and twist the very passages of Scripture to do so.. just as he did to Eve in the Garden! You who struggle in this area of satan whispering that you've "crossed the line", know this: If you're honest with your sin and bondage and seek to walk in freedom - genuinely seek freedom! - you are not in the category of being a blasphemer. In fact, you are on your way to deliverance! Christians struggle and many are in bondage to sin and are seeking the freedom of Christ in their lives. This article is referring to those who love their sin, enjoy their sin and are hypocrites; it's these that are in danger of damnation and it's to these that the Scripture cries out against.