What Is The Unforgivable Sin (and can I be guilty of it)?

The Bible says blaspheming the Holy Spirit is the unforgivable or unpardonable sin. What does that mean? Could I be guilty of that sin?  Is it possible that I have accidently committed it?  These are common questions and here I provide a reassuring answer.  But this episode closes with a couple of challenges the Bible provides us relative to the Holy Spirit on which Christians might need to focus more attention.