Q. What is the unpardonable sin, or the sin that Jesus said "shall not be forgiven?" I am afraid that I have committed it.
A. I think that just about everyone working in Christian ministry has been contacted by Christians concerned that they have committed the unpardonable sin. Let's see what this sin is and determine whether it is possible for Christians to commit the unpardonable sin.
In Matthew 12:22-37 we read that the Pharisees accused Jesus of casting out demons by the power of Beelzebul (or Beelzebub). Among the things Jesus replies to them, He says, "Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven men. Whoever speaks a word against the Son of Man, it will be forgiven him; but whoever speaks against the Holy Spirit, it will not be forgiven him, neither in this age, nor in that which is to come" (Matthew 12:31-32, World English Bible, WEB—used throughout unless otherwise noted).
Blasphemy against the Holy Spirit, then, is the unforgivable or unpardonable sin. The man who was possessed by a demon was both "blind and mute" (verse 22). After Jesus healed him, "the blind and mute man both spoke and saw." This is an event that should have caused onlookers to praise God. It was a witness to the power of the Holy Spirit.
But, instead of giving glory to God, the Pharisees credited demons. In Mark's account of this same event, we see that the Pharisees themselves identify Beelzebub as "the prince of the demons" (Mark 3:22).
So, being given a clear witness of the power of the Holy Spirit, the Pharisees spoke a blasphemous lie to try to keep people from seeing the logic that would lead them to know that the miracle was from God (Jesus outlines the logic they should have used in Matthew 12:25-30 and Mark 3:23-27). They rejected the Holy Spirit, attributed His work to the devil (for another clear example, see Acts 7:51), and said, "He has an unclean spirit" (Mark 3:30). Jesus calls this a blasphemy against the Holy Spirit and says that it is unforgivable. In Matthew 12:32 He adds, "Whoever speaks a word against the Son of Man, it will be forgiven him; but whoever speaks against the Holy Spirit, it will not be forgiven him, neither in this age [that is, the Old Covenant age then present], nor in that which is to come [the New Covenant age we are now in]." Why? Why is sinning against the Holy Spirit different than sinning against the Son of Man?
Notice that in Matthew 12:32, Jesus says, "Whoever speaks a word against the Son of Man...." He does not say, Whoever speaks against the Son of God or against the Christ, because these titles would be admitting knowledge of who Jesus was. Blasphemy against the Son of God or against the Christ would be willful blasphemy against God. Jesus does not say that can be forgiven.
Notice 1 Timothy 1:12-13: "And I thank him who enabled me, Christ Jesus our Lord, because he counted me faithful, appointing me to service; although I was before a blasphemer, a persecutor, and insolent. However, I obtained mercy, because I did it ignorantly in unbelief." Paul says that, before he was saved, when he persecuted the assembly of God's saints, he was not among those who willfully acted against the Holy Spirit. What he did, he did ignorantly. He knew what the Gospel said but he did not believe it. But, as soon as he was presented with evidence that the Gospel was true (encountering the resurrected Christ), he repented and believed. From this, we can conclude that the sin committed by the Pharisees who saw Jesus cast the demon out of this man was neither done in ignorance nor in unbelief. Incredibly, those Pharisees who saw that miracle knew that it was worked by the power of God, yet they still fought against it, even lying by trying to convince others that it was done by demonic power. In other words, they chose to take a stand against God, to fight the Holy Spirit Himself.
The reason the sin of these Pharisees was unpardonable is because it was unrepentable. It is one thing to be like Paul. Paul had not met Jesus during His earthly ministry before His crucifixion. He had not seen Jesus' miracles. What he had heard was hearsay that could have been wrong. He thought that a doctrine was being taught that seemed to him to be in opposition to the religion he'd been brought up with. He was in ignorance of the identity of Jesus. But when he miraculously met Jesus on the road to Damascus, Jesus' identity was revealed to him and he quickly repented.
It is a totally different thing to see Jesus' miracles and to know that He is working these miracles by the Spirit of God and to purposely blaspheme the Spirit by saying He is a demon. Since it is God who grants repentance and the Holy Spirit or Counselor who convicts the world about sin (see John 16:8), someone who willfully fights against the Holy Spirit can never repent. They have declared themselves enemies of the only means anyone has of repentance—the conviction of sin granted by God that is a work of the Holy Spirit. There is no hope for such people. Jesus did not die for them. They are "as unreasoning creatures, born natural animals to be taken and destroyed" (2 Peter 2:12).
Sometimes we might feel that we are condemned by careless, wicked things we have said. Jesus is not talking about such things. The unpardonable sin is the purposeful, premeditated opposition to work someone knows is from God. It is not possible for Christians who have believed on Jesus Christ as their Savior and repented through the conviction of the Holy Spirit and have the Holy Spirit dwelling in them to commit the unpardonable sin. John writes, "We know that whoever is born of God doesn't sin, but he who was born of God keeps himself, and the evil one doesn't touch him" (1 John 5:18). This is because, as Paul explains, "There is therefore now no condemnation to those who are in Christ Jesus, who don't walk according to the flesh, but according to the Spirit. For the law of the Spirit of life in Christ Jesus made me free from the law of sin and of death" (Romans 8:1-2).
The Pharisees who committed the unpardonable sin were not concerned about their sin. They had only evil intentions against God. If you are concerned that you have committed the unpardonable sin, then the very tenderness of your heart that causes you to be concerned shows that you have not committed it.