So the question is raised: What is the unpardonable sin that Jesus speaks of in Matthew 12 and can a Christian commit it?
(1) What the unpardonable sin is not: a) it is not murder (cf. Moses, David, Paul); b) it is not adultery (cf. David and Ps. 32:1-2a); c) it is not suicide (no other supportive Scripture).
(2) Context is decisive. The statement is made in the midst of Jesus' earthly life/ministry. The Pharisees had not merely been rejecting Jesus' work. They had been attributing to Satan the work of the Holy Spirit. They did so, not out of ignorance, but out of a conscious disputing of the indisputable.
(3) Therefore, blasphemy of the HS is willful, wide-eyed slandering of the work of the HS, attributing to the devil what is undeniably divine. These men had seen as well as anyone could see and had known as clearly as anyone could know that Jesus performed his miracles by the power of the HS, yet they defiantly insisted, contrary to what they knew to be true, that it was Satan who empowered him. This was not a one-time, momentary slip or inadvertent mistake in judgment, but a persistent, life-long rebellion in the face of inescapable truth. Blasphemy of the HS is not a careless act but a calloused attitude. The Pharisees had seen Jesus heal the sick, raise the dead, cleanse the lepers, teach the Sermon on the Mount, give sight to the blind, heal the paralytics. Blasphemy of the HS, therefore, is not just unbelief but unashamed unbelief that arises not from ignorance of what is true but in defiance of what one knows beyond doubt to be true. It is not mere denial, but determined denial; not mere rejection, but wanton, willful, wicked, wide-eyed rejection.
(4) Why is there greater guilt in blasphemy against the HS than in blasphemy against the Son of Man? It certainly isn't because one has greater dignity or glory than the other. Perhaps to reject Jesus during his earthly humiliation was forgiveable because his glory was veiled. To reject the power of the HS, as seen in his miracles, was unforgiveable because it was unmistakably divine; no doubts were possible. The distinction is between a failure to recognize the light and a willful rejection of the light once it is truly seen.
(5) Why is it unpardonable or unforgiveable? It isn't because there is a defect in Christ's atoning work. It isn't because God is incapable of granting forgiveness. This sin precludes pardon because by its very nature it precludes repentance. A sin of which one may repent is not the unpardonable sin. Blasphemy of the HS is by definition, unrepentant repudiation of the HS, unrepentant i