When we deny that the Holy Spirit is a third divine being we are committing the unpardonable sin. “Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men. And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come.” Mathew 12:31, 32. Why don't you stop promoting this horrible sin?

Answer:

This verse is often misunderstood and misapplied to mean that the unpardonable sin is rejecting the person of the Holy Spirit. But is this the correct interpretation? What does "blasphemy against the Holy Ghost" actually mean?
The context of Jesus' speech must not be overlooked. Let us read the passage in its setting:

“And Jesus knew their thoughts, and said unto them, Every kingdom divided against itself is brought to desolation; and every city or house divided against itself shall not stand: And if Satan cast out Satan, he is divided against himself; how shall then his kingdom stand? And if I by Beelzebub cast out devils, by whom do your children cast them out? Therefore they shall be your judges. But if I cast out devils by the Spirit of God, then the kingdom of God is come unto you. Or else how can one enter into a strong man's house, and spoil his goods, except he first bind the strong man? and then he will spoil his house. He that is not with me is against me; and he that gathereth not with me scattereth abroad. Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men.” Matthew 12:25-31

The parallel story is found in the Gospel of Mark. This sheds further light on this question:

“Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme: But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation: Because they said, He hath an unclean spirit.” Mark 3:28-30

In Mark 3:30 we read that the reason Jesus pointed the people to the unpardonable sin was “Because they said, He hath an unclean spirit.” After healing the blind and deaf the Pharisees accused Jesus of casting out devils by the power of Beelzebub (Matthew 12:24), in reply to their accusation Jesus said:
“All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men.”

What does accusing Jesus of performing miracles by the power of Beelzebub have to do with rejecting the Holy Spirit? What moved Jesus to say what He said?

It is clear that the sin against the Holy Spirit is committed when one knowingly and deliberately attributes the work of the Holy Spirit to Satan. When there is a determined and persistent resistance to truth and evidence, then one is sinning against the Holy Spirit. The unpardonable sin is not rejecting the person of the Holy Spirit, rather it is rejecting light and refusing to repent and heed the promptings of God's Spirit. By continually rejecting to repent and to accept the light that God has shown our way, we are blocking God's avenue to reach us. By placing ourselves where we can not be reached by God we commit the unpardonable sin in that we leave no way for God to reach us.

Spirit of Prophecy:
“The sin of blasphemy against the Holy Spirit does not lie in any sudden word or deed; it is the firm, determined resistance of truth and evidence (MS 30, 1890).” {5BC 1093.1}

“No one need look upon the sin against the Holy Ghost as something mysterious and indefinable. The sin against the Holy Ghost is the sin of persistent refusal to respond to the invitation to repent.” {FLB 58.8}

According to the inspired pen, the blasphemy against the Holy Ghost is the sin of persistent refusal to repent and determined resistance of truth and evidence.

We read in the Spirit of Prophecy:
“Those who made this charge against Christ were on perilous ground. They were quenching the last ray of light emanating from the throne of God to their prejudiced, benighted souls. Jesus warned them of their danger, saying: "All manner of sin and blasphemy shall be forgiven unto men; but the blasphemy against the Holy Ghost shall not be forgiven unto men. And whosoever speaketh a word against the Son of man, it shall be forgiven him; but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come." {ST, October 1, 1896 par. 5}

Again we read:
“By rejecting the light that was shining upon them, by refusing to examine the evidence to see whether the messages were from heaven, the Pharisees sinned against the Holy Ghost. Christ, the world's Redeemer, was in the world. "All things were made by him; and without him was not anything made that was made. In him was life; and the life was the light of men. And the light shineth in darkness; and the darkness comprehended it not. . . . He was in the world, and the world was made by him, and the world knew him not. He came unto his own, and his own received him not." The voice of his Spirit came to them, saying, "This is the Son of God; believe on him." But turning their faces from the light, they refused to listen, choosing, instead, to cultivate their unbelief. Thus the light which, if received, would have been to them a savor of life unto life, rejected, became a savor of death unto death,--death to spirituality.” {RH, July 27, 1897 par. 2}

“Brother P, you ask if you have committed the sin which has no forgiveness in this life or in the life to come. I answer: I do not see the slightest evidence that this is the case. What constitutes the sin against the Holy Ghost? It is willfully attributing to Satan the work of the Holy Spirit. For example, suppose that one is a witness of the special work of the Spirit of God. He has convincing evidence that the work is in harmony with the Scriptures, and the Spirit witnesses with his spirit that it is of God. Afterward, however, he falls under temptation; pride, self-sufficiency, or some other evil trait, controls him; and rejecting all the evidence of its divine character, he declares that that which he had before acknowledged to be the power of the Holy Spirit was the power of Satan. It is through the medium of His Spirit that God works upon the human heart; and 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. By denying the evidence which God has been pleased to give them, they shut out the light which had been shining in their hearts, and as the result they are left in darkness. Thus the words of Christ are verified: “If therefore the light that is in thee be darkness, how great is that darkness!” For a time, persons who have committed this sin may appear to be children of God; but when circumstances arise to develop character and show what manner of spirit they are of, it will be found that they are on the enemy‟s ground, standing under his black banner.” {Testimonies for the Church, vol. 5, p. 634}