What is the Unpardonable Sin?

Billy Graham's book on The Holy Spirit is one of the best I have ever read. It will shock you at times.

In his book, he gives the definition of the unpardonable sin. You know, Jesus said, "I tell you, every sin and blasphemy will be forgiven people, but the blasphemy against the Spirit will not be forgiven." Matthew 22:31 (ESV)

So what is the unpardonable sin? Graham says, "The unpardonable sin involves the total and irrevocable rejection of Jesus Christ. It is rejecting, completely and finally, the witness of the Holy Spirit, which declares that Jesus Christ is the Son of God who alone can save us from our sins. No one has committed the unpardonable who continues to be under the disturbing, convicting, and drawing power of the Holy Spirit. When a person has so resisted the Holy Spirit that He strives with him no more, then there is eternal danger." (Page 124).