Answer
In the immediate context of Luke 12:10, there is not much to indicate specifically what Jesus meant by "blasphemy of the Holy Spirit." It seems to be somewhat paired, at least in a rhetorical way, with Luke 12:8-9. Both Luke 12:8-9 and Luke 12:10 speak of those who will be saved versus those who will not be saved. If we see Luke 12:8-9 as a parallel to Luke 12:10, such that Luke 12:10 repeats the same ideas as Luke 12:8-9 with different words, then we might easily come to the idea that to blaspheme the Holy Spirit is to deny Christ before men. The fact that the subject of the second phrase of the first pair ("he who denies me before men...") is conceptually very similar to the subject of the first half of the second pair ("he who speaks a word against the Son of Man...") makes this approach very tempting, as does the correspondence between the predicates of the second half of the first pair ("will be denied before the angels of God") and the second half of the second pair ("it will not be forgiven him"). This intepretation may be further supported by the fact that the Holy Spirit bears witness to Christ, so that to deny Christ is to reject the witness of the Holy Spirit.
Though this may seem like a good option at first, there are some problems with it that, in my opinion, are too significant to overcome. It is simply not the case that the sin of denying Christ before men is unforgiveable, neither is the sin of rejecting the witness of the Holy Spirit unforgiveable. The Scriptures provide explicit proof of this in the person of Paul (Acts 8:1-3; 9:1-20). After Paul had denied Christ, implicitly also denying both the works that Christ had done, and the Spirit through whose power he had done them, and also after he had punctuated these denials by persecuting those who confessed Christ, Paul came to faith and was forgiven. Yet, Jesus made it clear that blasphemy of the Holy Spirit will not be forgiven either in this age or in the age to come (Matt. 12:32). Some respond to this objection by saying that blasphemy of the Holy Spirit is limited to final rejection of Christ, not just initial rejection. The problem with this response is that final rejection does not differ from initial rejection in any qualitative or quantitative sense. To all analysis, final rejection is the same sin as initial rejection. The only difference is the coincident death of the person. But a person's death does not alter the nature of his/her sin of rejecting Christ.
Further, although we clearly have a similar rhetorical structure between Luke 12:8-9 and Luke 12:10, treating it as such a close literary parallelism that we interpret one by the other is not as helpful as it might first seem. Specifically, to do so is to equate denying Christ with blaphemy of the Holy Spirit, but not with speaking against Christ. It is rather hard to imagine that denying Christ is not a form of speaking against Christ - that's a hair I don't feel comfortable splitting. I would prefer to see the literary similarity as indicating the eternal consequences of our actions on earth, but not as indicating an identity between denying Christ and blaspheming the Holy Spirit. Besides, the actual nature of Hebrew parallelism is that it rarely says the same thing with different words. The typical idea behind what is commonly called "synonymous parallelism" is "A and what's more B." That is, the "parallel" phrase does not say the same thing. Rather, it says something related but different. This would argue against equating "A" (denying Christ) with "B" (blaspheming the Holy Spirit).
On the issue of sins that do and don't lead to death, in my estimation Paul and John may be speaking of different kinds of death. Paul is speaking of eternal death as a spiritual state (in contrast to eternal life [Rom. 6:23]). John, however, may be speaking merely of the physical death of the body (cf. James 5:14-16). Also, whereas Paul speaks of sin that leads to death for the unbeliever, John appears to be speaking of sin that leads to death for a "brother," one who short of evidence to the contrary is assumed to be saved. When believers sin, our sins do not lead to spiritual death because we are forgiven in Christ (e.g. Rom. 8:34; 1 John 2:1-2). Even if John is speaking of eternal death, his point may be that Christians ought not to pray for a so-called brothers who reveal that they are truly unsaved and without hope of salvation (i.e. via blasphemy of the Holy Spirit).
Answer by Ra McLaughlin
Ra McLaughlin is Vice President of Finance and Administration at Third Millennium Ministries.