Today, if you hear his voice, do not harden our hearts as in the rebellion. (Hebrews 3:15)
What is the unpardonable sin? The sin against the Holy Spirit. It consists in willfully attributing to Satan the work of the Holy Spirit.
Sounds rather preposterous, doesn’t it? But it’s very possible for alleged Christians to commit this sin and not know it.
Suppose that in your life you observe the workings of the Holy Spirit. It convicts you of your sin, and shows you the truth. Were there had been darkness in your life there is now light. This is the special work of the Spirit of God, as it is written
And when [the Holy Spirit] comes, he will convict the world concerning sin and righteousness and judgment…When the Spirit of truth comes, he will guide you into all the truth.(John 16:8,13 ESV)
“The Holy Spirit witnesses in you that the work being done is of God. Afterward, however, you fall under temptation: pride, self-sufficiency, or some other evil trait, controls you; and rejecting all the evidence that you had had before of the convicting power being of divine character, you declare that that which you had before acknowledged to be the power of the Holy Spirit was not real after all [You, in short, deny the evidence which God had shown you, and attribute it to something—anything—else]. You shut out the light which had been shining in your heart leaving you in darkness.” And so you continue in your sin, stifling the still small voice inside you.
Why is this unpardonable? Is it that God is especially appalled at us when we deny his work or ignore his advice? No. God isn’t petty. The problem lies in us.
“It is through the medium of His Spirit that God works upon the human heart; and when men and women willfully reject the Spirit they cut off the channel by which God can communicate with them.” If you reject the Holy Spirit, will you be able to repent of your sin? How is God to continue to work in your life?
The Jewish leaders in Jesus’ time
While the life of Jesus and his mission to save us is depicted with vivid clarity in the Bible, a dark story line threads throughout the gospels: the story of a group of people who willfully rejected their savior. The Pharisees and the other religious leaders of the Jewish nation rejected Jesus even after he performed miracle after miracle. Where common uneducated folk acknowledged his divinity, the Jewish leaders could not accept that the savior would come in such humble guise, and that the promised one would find fault with them. They could not accept his claim to divinity, and willfully thrust aside the convicting evidence aside as the work of Satan (Matthew 9:34).
“Jesus, who was touched by human woe, who healed the sick, restored sight to the blind, hearing to the deaf and speech to the dumb, who fed the hungry and comforted the sorrowful, was driven from those he had labored to save. He who walked upon the heaving billows, and by a word silenced their angry roaring, who cast out devils that in departing acknowledged Him to be the Son of God, who broke the slumbers of the dead, who held thousands entranced by His words of wisdom, was unable to reach the hearts of those who were blinded by prejudice and hatred, and who stubbornly rejected the light.”
As for us, let us not harden our hearts against the convicting power of the Holy Spirit, but ask day by day for the strength that is needed to resist the enemy. If we do, God—though the Holy Spirit—will work in us both to will and to work for his good pleasure (Phil 2:3).
 E.G. White, Counsels for the Church: “Christians to Represent God”
 E. G. White, Desire of Ages: “Priestly Plottings”