The unpardonable sin is to blaspheme against the Holy Ghost. Notice the similarities between what Sherem did and the language used in DC 76 to describe the sons of Perdition,
'Those who know my power…and suffered themselves through the power of the devil to be overcome, and to deny the truth and defy my power—…
Having denied the Holy Spirit after having received it, and having denied the Only Begotten Son of the Father, having crucified him unto themselves and put him to an open shame' (DC 76:31,35).