The unforgivable sin is in regard to the blasphemy against the Holy Spirit, which is mentioned in Matthew 12:22-32 and Mark 3:22-30. This case of blasphemy which is unpardonable is a specific one.
In Matthew 12:31-32, the Pharisees having witnessed irrefutable proof that Jesus was casting out demons in the power of the Holy Spirit, claimed instead that Jesus had an unclean spirit and casts them out by Beelzebul the ruler of the demons. (Matthew 12:24; Mark 3:30).
Jesus was specific about what they did to commit "blasphemy against the Holy Spirit" and these Pharisees who committed this unforgivable sin were not believers. What is unpardonable in general is the state of continued unbelief. (John 3:18)