Jesus says that there is ONE sin that is completely unpardonable/unforgivable for all of eternity, and that the sin He is referring to is blasphemy of the Holy Spirit. Episode #4 of the Bible Questions Podcast looks through church history and the Word of God to answer the question: What exactly is the unforgivable sin? These questions are also addressed:

How do I know if I have committed the unpardonable sin?

Why did Jesus even mention that there was an unpardonable sin?

Are Beelzebul/Beelzebub and Sata