Just think how much worse the punishment will be for those who have trampled on the Son of God, and have treated the blood of the covenant, which made us holy, as if it were common and unholy, and have insulted and disdained the Holy Spirit who brings God's mercy to us.
- Hebrews 10:29
The work of the Holy Spirit in the lives of nonbelievers is to make them aware of their need for Jesus, to convict and convince them of their sin. Stephen, when he spoke to the unbelieving Sanhedrin, said, "You stubborn people! You are heathen at heart and deaf to the truth. Must you forever resist the Holy Spirit? That's what your ancestors did, and so do you!" (Acts 7:51).
Saying no to the work of the Holy Spirit is, in effect, resisting Him. That is what Stephen was saying to these spiritual leaders: You are resisting the Holy Spirit.
Resisting the Holy Spirit can then lead to insulting the Holy Spirit, which is a sin nonbelievers can commit. Jesus said of the Holy Spirit, "And when he comes, he will convict the world of its sin, and of God's righteousness, and of the coming judgment" (John 16:8). The Spirit has come to show us our need for Jesus. He awakens us to our sinful condition. And He helps us realize that the death of Jesus on the cross is sufficient to cause us to come into a relationship with God, if we will turn from our sin and put our faith in Him.
But when people refuse the working of the Holy Spirit in this way, when they say they don't need Jesus Christ, that is insulting the Holy Spirit. Hebrews 10:29 says, "Think how much more terrible the punishment will be for those who have trampled on the Son of God and treated the blood of the covenant as if it were common and unholy. Such people have insulted and enraged the Holy Spirit who brings God's mercy to His people."
What a fearful thing to insult or enrage the Holy Spirit. And it starts with resisting Him.