The "unpardonable sin" in the New Testament is to deny the work of the Holy Spirit when confronted with its clear and obvious working. So the authorities in Jesus' day committed the unpardonable sin when Lazarus was raised from the dead and because of that they plotted to kill Jesus. [see John 11:45-53]
Matthew 12:32 Anyone who speaks a word against the Son of Man, it will be forgiven him; but whoever speaks against the Holy Spirit, it will not be forgiven him, either in this age or in the age to come.
I hope you have found this helpful.