The Pharisees attributed the power of Jesus Christ to demons (Matthew 12:24; Mark 3:30). In other words, the Jewish leaders had outrightly rejected verifiable evidence that He was sent by God. That is the precise identity of what has been termed the unpardonable sin.
This scenario is not one that can be replicated today because nobody has seen Christ publicly performing miracles. He is presently sitting at the right hand of the Father (Ephesians 1:20; Colossians 3:1). Thus, nobody can actually commit this form of blasphemy against the Holy Spirit.
On the other hand, persistent unbelief does not receive the forgiveness of God. A person who dies in a state of voluntary opposition to the conviction of heart brought about by the Holy Spirit will be eternally condemned. That sin in a sense is unforgivable. We must repent and place our trust in Christ's work alone for salvation (John 3:16).