What is the unpardonable sin?

Three of the four gospels mention the unpardonable or “unforgiveable” sin—Matthew, Mark and Luke. Matthew’s gospel tells it this way: He who is not with me is against me, and he who does not gather with me scatters. And so I tell you, every sin and blasphemy will be forgiven men, but blasphemy against the Spirit will not be forgiven. Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come—Jesus Christ, Matthew 12:30-32. Mark’s account adds some additional information: I tell you the truth, all the sins and all the blasphemies of men will be forgiven them. But whoever blasphemes against the Holy Spirit will never be forgiven; he is guilty of an eternal sin.” He said this because they were saying, “He has an evil spirit”—Mark 3:28-30. The context is really important in understanding what Christ is responding to. He had performed some amazing miracles, miracles done in the power of the Holy Spirit. But his opponents, largely the religious establishment, even as they witnessed these amazing events, attributed His power to Satan; for instance: Then they brought him a demon-possessed man who was blind and mute, and Jesus healed him, so that he could both talk and see. All the people were astonished and said, “Could this be the Son of David?” But when the Pharisees heard this, they said, “It is only by Beelzebul, the prince of demons, that this fellow drives out demons”—Matthew 12:22-24. There are two possible meanings: (1) in the context of the time, that those people who chose to credit Satan with the work of the Spirit rather than acknowledge Jesus for Who and What He was had hardened themselves spiritually beyond the power of repentance, and/or (2) inclusive of our times, that those who consciously reject the work of the Spirit of God cannot find forgiveness. This second option would obviously include anyone who rejects the call of Christ to surrender to Him. That would also include those who deny who Jesus is; in fact, the apostle John referenced those in His first letter: [Y]ou have an anointing from the Holy One, and all of you know the truth. I do not write to you because you do not know the truth, but because you do know it and because no lie comes from the truth. Who is the liar? It is the man who denies that Jesus is the Christ. Such a man is the antichrist—he denies the Father and the Son. No one who denies the Son has the Father; whoever acknowledges the Son has the Father also—I John 2:20-23. John goes on in his letter to say, Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world. This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the antichrist, which you have heard is coming and even now is already in the world. You, dear children, are from God and have overcome them, because the one who is in you is greater than the one who is in the world. They are from the world and therefore speak from the viewpoint of the world, and the world listens to them. We are from God, and whoever knows God listens to us; but whoever is not from God does not listen to us. This is how we recognize the Spirit of truth and the spirit of falsehood—I John 4:1-6. John included a final note on the nature of antichrist in his second letter: Many deceivers, who do not acknowledge Jesus Christ as coming in the flesh, have gone out into the world. Any such person is the deceiver and the antichrist—II John 7. One of the main ministries of the Holy Spirit is simply declaring the truth of who Jesus is; as Peter acknowledged, You are the Christ, the Son of the living God—Peter, Matthew 16:16b. How did he come to that belief? Jesus tells us: [T]his was not revealed to you by man, but by My Father in heaven—Jesus Christ, Matthew 16:17b. Rejecting the identity of Jesus Christ rejects the work of the Spirit as well as the work of Jesus. A person coming to that place will not be forgiven simply because they are rejecting the only means by which forgiveness can be experienced—the work of Christ on our behalf. Hell will be populated by all those who have rejected the work of the Spirit in revealing to them who Jesus Christ truly is.