Hello NewLifeinChrist, I believe that the unpardonable sin (blasphemy against the Holy Spirit) is never done in ignorance of the truth. Rather, it's committed by those who (like the Pharisees) fully understand that what they are uttering against the Spirit is a lie, a lie that requires such an utterly cold, black heart to commit that those who commit it could care less that they are doing so.
So, for MacArthur (or anyone else) to simply make "the suggestion" that people who believe that they are hearing from God "may be" hearing from Satan or a demon instead, is something that wouldn't qualify as a blasphemy, much less as the unpardonable sin in the Bible.
We do, after all, have Biblical warnings about Satan (e.g. that he can appear to us as "
an angel of light" ..
2 Cor 11:14, and will try to deceive us, if possible), as well examples of the very thing that MacArthur is talking about occurring fairly recently in real life. For instance, a few years ago theologian Gleason Archer, who was fluent in the language that was being shouted in tongues from the pulpit one Sunday (by a longtime member of his congregation), refused to repeat what he heard that morning because, rather than praises/truth/thanksgiving about/given to the Godhead, the words that his fellow congregant was uttering in the service were actually extreme blasphemies against the Holy Spirit, instead
God bless you!!
--David