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This is possibly a reaction against popular Gnosticism of the time.  It was difficult for Yesu’s followers to be accommodating toward the Gnostics who were in many respects closely allied because of the common use of Wisdom literature.  The most striking instance where the Gnostics deviated from Yesu was in their denigration of the Male aspect of God the Creator as the Demiurge, which they posited as the evil one, almost styled according to the Zoroastrian Ahriman.  Their utilisation of Wisdom (Sophia) as the divine Female Power was often-times regarded as blasphemous compared when to the doctrines of the eastern Way of Yesu.  Yesu seems to say that even if you do not understand exactly who and what God the Father and God the Mother is – you do always understand Who is meant when you say ‘Holy Spirit’.  Thus, to mistakenly speak blasphemous of the Male or Female aspects can be forgiven – but to blaspheme the Holy Spirit cannot be forgiven.
See Mark 3:28-30 and Matthew 12:31-2 for the Synoptic use of this Saying.  See John 5:23 for the Johannine usage.