Someone recently asked me what is the blasphemy of the Holy Spirit. Years ago, I even counseled with someone who thought she had committed this sin. It is a perennial question for Bible readers. The relevant passages are Matthew 12:24-32, Mark 3:22-30, and Luke 11:15-23, 12:10.
Jesus is accused of casting out demons by the power of Beelzebul (other manuscripts spell it Beelzebub). It is clear from the context that Beelzebul is another name for Satan. Beelzebul is “the prince of demons” (Matthew 12:24). The parallel of “if Satan casts out Satan” with “by Beelzebul … this man casts out demons” also makes this clear. Beelzebul is another name for Satan.
What is blasphemy? It is “speech that denigrates or defames, reviling, denigration, disrespect, slander.”1 In these passages, the blasphemy of the Holy Spirit is attributing the work of the Spirit to Satan. So the conditions for this sin to occur are: (1) Jesus performs a miracle by the power of the Holy Spirit, and (2) someone ascribes the work of the Holy Spirit in this miracle to the work of Satan. It would seem from context that you would have needed to have been present at Jesus’ ministry and witnessed a miracle to commit this particular sin.
Why is this an unforgivable sin? The miracle is a pointer to Christ’s identity. The person who has become so hardened that he believes Christ is in league with Satan will not come to Christ for salvation. Since salvation is found only in Jesus (Acts 4:12), there can be no forgiveness for such a person. F.F. Bruce notes one other factor: “The answer seems to be that the nature of this sin is such that one does not repent of it, because those who commit it and persist in it do not know that they are sinning.”2
Jesus reasons with his accusers. Every kingdom divided against itself will be left desolate, so it doesn’t make sense that Satan would cast out Satan. By what do your sons cast out demons, therefore they will be your judges. The casting out of demons is evidence of the Kingdom of God. The plundering of Satan’s possessions is evidence that someone stronger than Satan has arrived on the scene. This reasoning with his accusers suggests to me that maybe even they have not yet reached the unforgivable stage, but they are in danger of it. A settled opposition to Christ would leave them without hope of forgiveness.
The bottom line is this: if you have some sensitive soul who is fearful of having committed this sin, but also desires to repent, then they definitely have not committed this sin. But this passages warns us to guard our hearts. It is possible to harden our hearts so much that we are no longer responsive to the call to repentance.
1“βλασφημία,” BDAG, 178.
2F.F. Bruce, Hard Sayings of Jesus, p. 90