The Trinity teaching that the Holy Spirit is a separate person is a false and erroneous teaching. As demonstrated in our free study guide Is God a Trinity? this belief is not found in the Bible, but is based in ancient religions that long predate Christianity. So rejecting this false teaching could not be an act of blasphemy.
The words of Jesus Christ in Matthew 12:32 Matthew 12:32And whoever speaks a word against the Son of man, it shall be forgiven him: but whoever speaks against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come.
American King James Version×are: “Anyone who speaks a word against the Son of Man, it will be forgiven him; but whoever speaks against the Holy Spirit, it will not be forgiven him, either in this age or in the age to come.” A similar statement is found in Mark 3:28-29 Mark 3:28-29  Truly I say to you, All sins shall be forgiven to the sons of men, and blasphemies with which soever they shall blaspheme:  But he that shall blaspheme against the Holy Ghost has never forgiveness, but is in danger of eternal damnation.
American King James Version×.
At any time, God can and will forgive the sin of departing from Him, if the sinner wants forgiveness and genuinely repents.
Someone with insufficient knowledge of Jesus who may have been adversely influenced by biases, false accusations and the claims of jealous leaders may have looked on Him as a mere man. They might have stated things about Him that were untrue and disrespectful. Some without understanding of God the Father and Jesus Christ might even have made vile statements against them. Jesus says these sins would be forgiven on repentance.
But in the context of Matthew 12, the Pharisees were confronted with a miracle clearly demonstrating the power of God. Jesus had banished a demon from a man. They dismissed it as Christ having done this miracle by the power of Satan. But Jesus explained clearly that only by the power of a greater Spirit—that of God—could a demon be cast out.
They remained unmoved and hardened in their hearts. For them to see a clear demonstration of God’s power through His Spirit, yet still reject it, shows their attitude of willfully resisting the Holy Spirit, and this is why Jesus rebuked them with this grave warning. The Holy Spirit is the means through which God acts on and within our hearts and minds to change us into the kind of people He wants us to be. Blasphemy against the Spirit involves a refusal of that power that allows us to change.
The book of Hebrews addresses how the attitude of rejecting the work of the Holy Spirit can manifest itself among believers through spiritual neglect and persistence in sin: “For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit, and have tasted the good word of God and the powers of the age to come, if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame” (Hebrews 6:4-6 Hebrews 6:4-6  For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
 And have tasted the good word of God, and the powers of the world to come,
 If they shall fall away, to renew them again to repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
American King James Version×; see also Hebrews 10:26-27 Hebrews 10:26-27  For if we sin willfully after that we have received the knowledge of the truth, there remains no more sacrifice for sins,  But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries.
American King James Version×).
The faithful and true believer will acknowledge, confess and repent of his or her sins, and ask for forgiveness. When we do this, we can take comfort in knowing that these sins do not lead to the second death because we are taking part in the conversion and overcoming process. John brings this out near the beginning of his letter in 1 John 1:8-9 1 John 1:8-9  If we say that we have no sin, we deceive ourselves, and the truth is not in us.
 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
American King James Version×. The apostle Paul acknowledged that he committed blasphemy and was guilty of opposing and rejecting Christ and persecuting His followers. But he repented of this and through God’s grace was forgiven (1 Timothy 1:13-14 1 Timothy 1:13-14  Who was before a blasphemer, and a persecutor, and injurious: but I obtained mercy, because I did it ignorantly in unbelief.  And the grace of our Lord was exceeding abundant with faith and love which is in Christ Jesus.
American King James Version×).
In contrast, those who come to the point of finally rejecting God will not seek to overcome sin through the power of His Spirit. In choosing not to change or repent of their sins, they are ultimately choosing death. That is the “unforgivable” sin—sin that someone refuses to repent of.
But we can take heart, because so long as we desire to repent and get our life back in order spiritually, God will always forgive. At any time, God can and will forgive the sin of departing from Him, if the sinner wants forgiveness and genuinely repents as we read in 1 John 1:9 1 John 1:9If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
American King James Version×: “If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness.”
Clearly, God will extend mercy to all who sincerely repent and change—regardless of their sin. Christ’s words, coupled with the warnings in Hebrews, are sobering. Nonetheless, we should hold fast to the hope that God will grant us repentance when we turn to Him.
To learn more about how God’s Spirit works in our lives, be sure to read “The Holy Spirit: God’s Power at Work” and download or request our free study guide Transforming Your Life: The Process of Conversion.