There are many sins against the Holy Spirit, but when people ask about the sin against the Holy Spirit, they are asking (and often worrying) about the sin that cannot be forgiven. There are three places in the Bible where Jesus talks about this sin. They basically say the same thing. In Matthew, we read: “But when the Pharisees heard it, they said, “It is only by Beelzebul, the prince of demons, that this man casts out demons.” In answer to that, Jesus says: “I tell you, every sin and blasphemy will be forgiven people, but the blasphemy against the Spirit will not be forgiven. And whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come” (Matthew 12:24-32).
Blasphemy against the Holy Spirit
This passage makes clear what the sin against the Holy Spirit is. It is blasphemy against the Holy Spirit. Jesus cast out devils through the power of the Holy Spirit, but the Pharisees denied this. They said Jesus did not work through the power of the Holy Spirit, but through the power of Satan. But they knew better than that! They willfully decided not to acknowledge the power of God.
Hardening of hearts
Jesus says this will never be forgiven. The reason for that is not that God would deny these people when they would ask for forgiveness. The reason is that they have hardened their hearts so much, that it has become impossible for them to repent and believe. A person who has blasphemed the Holy Spirit becomes obstinate, and refuses to see his sin as sin. He becomes impenitent: he has decided he is not going to repent. He resists the truth, even though deep down he knows it is the truth. He becomes presumptuous: he thinks he can receive salvation without repentance.
Tasted the Holy Spirit
The situation of a person like this is like the one described in Hebrews 6:4-6: “For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come, and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt.”
They have tasted the Holy Spirit and spit him out. That is blaspheming the Holy Spirit. That is telling God: ‘I know who You are, and I don’t want You.’ It is turning your back on God and never coming back.
That is the reason why many people have said, with good reason: ‘If you are afraid you have committed the sin against the Holy Spirit, you have not.’ Only those who have hardened their hearts against God, have committed the sin against the Holy Spirit.