C.S. Lewis penned a famous essay that suggests we really only have three options when it comes to forming an opinion about Jesus: that He is either liar, lunatic, or Lord. The verdict we reach could scarcely be more important.
Lewis' logic was chiefly wielded against those who claim Jesus to be a good man, a prophet, a teacher - but nothing more. Interestingly, we don't see that judgment placed upon Jesus by anyone during His earthly life and ministry!
In Mark 3, we see each of Lewis' possible conclusions expressed by varying groups of people:
There were those who were concerned that He might be a lunatic. "When his family heard it, they went out to restrain him, for people were saying, 'He has gone out of his mind.'" (vs. 21) While Jesus may not have had his family's support during His time on earth, later they came to believe in Him.
There were those who believed Him to be Lord. Oddly enough, at this point in the Gospel of Mark, it is only the demons who unequivocally hold this opinion! "Whenever the unclean spirits saw him, they fell down before him and shouted, 'You are the Son of God!'" (vs. 11)
Finally, there were those who believed Him to be a liar, in league with Satan. This was the judgment of the religious establishment. "And the scribes who came down from Jerusalem said, 'He has Beelzebul, and by the ruler of the demons he casts out demons.'" (vs. 22)
Making the wrong decision regarding the quality of Jesus' Spirit has the potential to imperil your soul. In response to the scribes' statement that His power came not from God but from the prince of demons, Jesus proclaimed, "Truly I tell you, people will be forgiven for their sins and whatever blasphemies they utter; but whoever blasphemes against the Holy Spirit can never have forgiveness, but is guilty of an eternal sin." (vs. 28-29)
Why? Why is everything else in this world forgivable, but blasphemy against the Holy Spirit is the ultimate sin? I have heard it explained that, in order to receive the forgiveness that Jesus offers, we must first believe that He is acting on behalf of God. We have to look upon His life and consider it good and deem Him holy. If we refuse Him and reject His Spirit on the basis that we believe Him to be demonic and evil rather than of God, there is no further route of salvation from that blasphemy. We have sawed off the branch of grace God has given us to cling to. We have sunk the lifeboat of deliverance He has provided. We have scorned His Spirit and rejected His Christ. What can possibly save us then, if we turn our back on the One Who came to save?