Blasphemy against the Holy Spirit for Non-trinitarians

First, look at Romans 8 in more detail:

Now, in that context, consider Matthew 12:31–32:

So I tell you, every sin and blasphemy can be forgiven …
Anyone who speaks against the Son of Man can be forgiven …

Jesus is willing to forgive all sins, even those that are explicitly against himself. All that is required in order to receive that forgiveness is that one truly repent of that sin (alter character to avoid it happening again) and that one ask Jesus for his forgiveness.

… except blasphemy against the holy spirit, which will never be forgiven.
… but anyone who speaks against the holy spirit will never be forgiven, either in this world or in the world to come.

Those that reject God's holy spirit are no longer "controlled by the holy spirit". They have chosen to reject God's guidance, and chosen instead to follow their sinful nature. They themselves have chosen to reject God's offer of salvation and immortal life. They have rejected God's only mechanism for forgiveness, so there is no way remaining for them to be saved.

Note that this is not a case of God punishing them for rejecting his holy spirit.
This is a case of receiving the natural consequences of their own deliberate bad choice.

Key Text

³¹Wherefore 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. ³²And 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. (Matthew 12:31-32, KJV; cf. Mark 3:28-29)

What is "Blasphemy"?

While you may not be asking what blasphemy means, clear definitions are important to a proper understanding of the question and its answer. According to our modern dictionary, "blasphemy" is given as:

blasphemy | ˈblasfəmē |
noun (plural blasphemies)
the act or offense of speaking sacrilegiously about God or sacred things; profane talk: he was detained on charges of blasphemy | screaming incomprehensible blasphemies.

The Greek word is defined in Strong's Concordance as:

βλασφημία blasphēmía, blas-fay-me'-ah; from G989; vilification (especially against God):—blasphemy, evil speaking, railing.

Essentially, blasphemy has broad application, and, as used in this answer, entails any form of speaking against God, including undermining His character or misrepresenting Him.

Who is the "Holy Spirit"?

Is the Holy Spirit a "person"? While the Bible may not define "person" or its usage, the Bible is clear as to the fact that God is both holy and a spirit.

Exalt the LORD our God, and worship at his holy hill; for the LORD our God is holy. (Psalm 99:9, KJV)

God is a Spirit: and they that worship him must worship him in spirit and in truth. (John 4:24, KJV)

God is both holy and a spirit--a "Holy Spirit." If God is a person, and God is a holy spirit (i.e. the "Holy Spirit" is God), it follows that the Holy Spirit is a person, and to speak against God's holy person is the epitome of "blasphemy."

Throughout the Bible, the Holy Spirit is used with respect to the spirit of God, the spirit of the Father, or the spirit of Christ Jesus, in whom the Father dwelt. Because the Father is a spirit, and that spirit was in Christ, the "holy spirit" represents God's own omnipresence, and is not human but divine.

Who is the "Son of Man"?

Jesus called himself the "son of man."

When Jesus came into the coasts of Caesarea Philippi, he asked his disciples, saying, Whom do men say that I the Son of man am? (Matthew 16:13, KJV)

This had deep Biblical significance with respect to his identity.

God is not a man, that he should lie; neither the son of man, that he should repent: hath he said, and shall he not do it? or hath he spoken, and shall he not make it good? (Numbers 23:19, KJV)

And also the Strength of Israel will not lie nor repent: for he is not a man, that he should repent. (1 Samuel 15:29, KJV)

What is Jesus Saying?

Jesus said that if one were to speak against the "son of man," it would be forgiven. Since Jesus was himself the "son of man," speaking against Jesus can be forgiven.

But to speak against the Holy Spirit is to speak, not against a mere man, but against God Himself. This, according to Jesus, is not forgiven.

Applications to Trinitarianism / Nontrinitarianism

As shown above, the Bible is clear that God is not a man. It is also clear that Jesus is a man. Anytime that a man speaks as if he were God, this is reckoned as blasphemy. The Jews, for example, accused Jesus of blasphemy for this very reason.

³²Jesus answered them, Many good works have I shewed you from my Father; for which of those works do ye stone me? ³³The Jews answered him, saying, For a good work we stone thee not; but for blasphemy; and because that thou, being a man, makest thyself God. (John 10:32-33, KJV)

The Jews were looking at Jesus' humanity, not recognizing the Divinity within him, which was speaking through him. When Jesus forgave sins, for example, it was not his humanity speaking--but the Spirit of God within him who gave him the words.

It is blasphemy for a man to speak as God. But Jesus did not do this. He spoke for God, not as if he were himself God. He never claimed to be God. He stated plainly that he spoke the Father's words, because the Father dwelt in him.

For I have not spoken of myself; but the Father which sent me, he gave me a commandment, what I should say, and what I should speak. (John 12:49, KJV)

Believest thou not that I am in the Father, and the Father in me? the words that I speak unto you I speak not of myself: but the Father that dwelleth in me, he doeth the works. (John 14:10, KJV)

If it is blasphemy for a man to say he is God, or to attempt to speak as if he were God, it is also blasphemy to speak as if another man were God. To call any man God, when God says He is not a man, is blasphemy.

"God the Son": Trinitarians have a "God the Son" to whom they give homage. This "God" is never mentioned in the Bible. The expression "God the Son" exists nowhere in the scriptures. God says He is not the "son of man"--whereas Jesus, whom Trinitarians call "God the Son," says he is the "son of man." In this, they essentially call God a liar, saying that God is the "son of man" even though God claims otherwise. This is the epitome of blasphemy.

To avoid blasphemy, one must not misrepresent God. God says He is not a man, and that He is not the son of man. If one portrays God as having lied in saying these things, one blasphemes.

"God the Holy Spirit": Further, Jesus taught that the Father is "the only true God" (see John 17:1-3). If one claims that the "Holy Spirit" is another "true God," other than the Father (as Trinitarians believe), then one likewise misrepresents God, and blasphemes.

Nontrinitarians are clear of this blasphemy, because they accept God's words at face value. God says what He means, and He means what He says.

I view the Spirit as the power of God, and the literal spirit of God; hence, "Spirit of God" in scripture.

Paul shows that God has a spirit [Rom 8:11, 1 Thes 4:8] and Jesus has a spirit [Rom 8:9, Phi 1:19]. Paul seems to even equate these spirits as a shared spirit [Rom 8:9, Gal 4:6].

It is stated that the Spirit resurrected Christ [Rom 8:11, 1 Pet 3:18]. It is stated numerous times that God (the Father) resurrected Christ [Acts 2:24, 32, 3:15, 26, 13:30-37, Rom 10:9, 1 Cor 6:14, Gal 1:1, Col 2:12]. This shows further evidence of the Spirit simply being an attribute/property of God.

God does everything by His Spirit. So to blaspheme the Spirit is to deny the power and diminish works of God. You're not blaspheming the Spirit as a person, but blaspheming who/what it represents, namely God. Similar to how blaspheming the name of the Lord is to blaspheme God [Lev 24:16, Rev 13:6].

Having to hand two large volumes of alphabetic biblical topics produced by one staunchly non-trinitarian group, I examined them to come up with an answer (though I disagree with this group's understanding on this, and other beliefs). Despite this, an objective answer can be given, fairly representing their view, purely by quoting from their own official literature.

First I looked under "Blasphemy". It correctly pointed out that the first one guilty of blasphemy was the one named Diabolos (meaning 'devil' or 'slanderer'). Then it went through blasphemy under the law covenant, then in the Greek scriptures. This was where the text in question was mentioned. Here is the explanation given:

Since Jesus was God's Son and direct representative, the things spoken against him may also properly be defined as blasphemy. (Lu 22:65) So, too, since the holy spirit or active force emanates from God and is intimately connected with God's person, Jesus could speak of "blasphemy against the spirit." This is stated to be the unforgivable sin. (Mt 12:31) Blasphemy is shown to originate within one's heart (Mt 15:19; Mr 7:21, 22); hence the heart condition, manifest in the willfulness involved, must relate to such blasphemy against the spirit. The incident that led to Jesus' statement concerning the unpardonableness of such sin demonstrates that it refers to opposing the operation of God's spirit. This would not be because of deception, human weakness, or imperfection; but the opposition would be willful and deliberate. The Pharisees clearly saw God's spirit at work in Jesus to accomplish good, yet for selfish reasons they attributed this power to Beelzebub, Satan the Devil, thereby blaspheming God's holy spirit. - Mt 12:22-23" Insight on the Scriptures Vol. 1, pp. 337-339, Watchtower Bible & Tract Society, 1988

Next, I looked for a "Holy Spirit" heading but that only said, "see SPIRIT". However, two pages previously, under the heading "Holiness" there was a sub-heading, 'Holy Spirit' which agreed with the quote above about 'holy spirit' [their way of writing this]. No personality is attributed to this 'active force' from Jehovah. This was further confirmed when I looked under the main heading "Spirit".

Its first sub-heading was, "Not a person"; the second was, "Personification does not prove personality"; the third was, "Lacks personal identification". Six pages on came the sub-heading, "Gaining and retaining God's spirit" where the last sentence in that short section repeated the previous quotation about blasphemy against that spirit, citing the text in question. (Ibid. Vol. 2, pp. 1017 - 1024)

This needs to be detailed, if ever so briefly, to let readers understand why the Watchtower Society speaks of blaspheming against an impersonal energy, or invisible power from Jehovah, by implying that one would actually be blaspheming against Jehovah God. It is not possible to blaspheme against any impersonal energy, such as electricity, or wind, or fire.

As the first quote in this answer shows at its start, one can blaspheme against the person of Jesus Christ (although they deny that he is God incarnate), and other quotes show that one can blaspheme against Jehovah God. But - technically - any charge of blasphemy against what they speak of as 'impersonal holy spirit' is actually blasphemy against Jehovah God, from whom this active force emanates.

As for the Romans 8:5-9 text in the question, their own NWT rendering of that text speaks of "God's spirit", and "Christ's spirit", as though those are two distinct things. This seems to be supported in verse 11 where it speaks of the spirit of him [i.e. God] who raised Christ from the dead dwelling in [anointed] Christians, and that same indwelling spirit of God making those believers' bodies alive.

It does not seem, as supposed in the question, that they "accept that the Holy Spirit is the [sic] both the Spirit of Christ and the Spirit of God" After all, they claim that Christ had a starting point in time, but Jehovah never did, and Jehovah has never been without "his holy spirit". Perhaps, however, they think in terms of Jehovah giving some of his holy spirit active force to Christ at a certain time. They do say this under the main heading, 'Christ':

"The personal name of Jesus followed by the title Christ may call attention to the person himself and that he is the one who became the Anointed One of Jehovah. This occurred when he reached about 30 years of age, was baptized in water, and was anointed with Jehovah's spirit visibly observed in the form of a dove descending upon him. (Mt 3:13-17) This is the point Peter made at Pentecost [quoting Acts 2:36-38]." (Ibid. Vol. 1, p. 438)

Further, under "God's Active Force: Holy Spirit" re. Matthew 28:19, believers being baptised in the name of the Father and of the Son and of the holy spirit:

"Hence baptism 'in the name of the holy spirit' implies recognition of that spirit as having its source in God and as exercising its function according to the divine will." (Ibid. Vol. 2, p. 1020, last sentence under sub-heading 'How baptized in its "name")

That might best be dealt with in a separate question as this point in the question risks deviating from the main question. However, the quotes serve to show how this particular non-Trinitarian group view 'holy spirit' and blasphemy against it.