What is the sin against the Holy Spirit?

Following my answer I will copy for you a couple of paragraphs from The Desire of Ages in which Mrs. White addressed the questions you have asked. (These paragraphs come from pages 321–323. You would find the whole passage, through page 325, enlightening.) You will see that she presents this sin as the resisting of the Holy Spirit’s call to us to repent and turn to God. Even in the face of the strongest evidence and appeals, we are free, if we wish, to refuse to yield to God. If we continually resist, eventually we won’t be impressed at all by the Spirit’s most powerful work on our behalf. We will have placed ourselves beyond God’s reach because He will not violate our will. God cannot forgive this sin because we refuse to bring it to Him or even to listen to His pleadings.

How can we know that we have not committed this sin? If we still feel the call of God to yield our life to Him, that is the work of the Holy Spirit on our heart. If we have been resisting His call, we can be thankful that He is still working on us, and we must not delay! We must make a full surrender of our life to the One who died for us. As He held nothing back, so we must hold nothing back. Delay or refusal is dangerous. Now is the accepted hour, the Bible says; now is the day of salvation.

It was just before this that Jesus had a second time performed the miracle of healing a man possessed, blind and dumb, and the Pharisees had reiterated the charge, “He casteth out devils through the prince of the devils.” Matt. 9:34. Christ told them plainly that in attributing the work of the Holy Spirit to Satan, they were cutting themselves off from the fountain of blessing. Those who had spoken against Jesus Himself, not discerning His divine character, might receive forgiveness; for through the Holy Spirit they might be brought to see their error and repent. Whatever the sin, if the soul repents and believes, the guilt is washed away in the blood of Christ; but he who rejects the work of the Holy Spirit is placing himself where repentance and faith cannot come to him. It is by the Spirit that God works upon the heart; when men willfully reject the Spirit, and declare It to be from Satan, they cut off the channel by which God can communicate with them. When the Spirit is finally rejected, there is no more that God can do for the soul. . . .

It is not God that blinds the eyes of men or hardens their hearts. He sends them light to correct their errors, and to lead them in safe paths; it is by the rejection of this light that the eyes are blinded and the heart hardened. Often the process is gradual, and almost imperceptible. Light comes to the soul through God’s word, through His servants, or by the direct agency of His Spirit; but when one ray of light is disregarded, there is a partial benumbing of the spiritual perceptions, and the second revealing of light is less clearly discerned. So the darkness increases, until it is night in the soul. Thus it had been with these Jewish leaders. They were convinced that a divine power attended Christ, but in order to resist the truth, they attributed the work of the Holy Spirit to Satan. In doing this they deliberately chose deception; they yielded themselves to Satan, and henceforth they were controlled by his power.