Our Lord makes a distinction between the physical and spiritual manifestation of the HolySpirit. He understands how people looking at His flesh and listening to His voice may say something against Him. But His HolySpirit by which He spoke is pure God. To reject and blaspheme His HolySpirit is unconsciounable.
The HolySpirit draws a person to faith in Christ. By offending the HolySpirit, they will never be drawn and so they will never be saved.