Your query opens up several important subjects. Let’s begin with the subject of the Holy Spirit. The Holy Spirit is “God Himself.” Now, He is distinct from the Father and the Son, being the third person of the Godhead. Nevertheless, He is God, as the Scripture makes abundantly clear (Matthew 28:19; Acts 5:3-4; 2 Corinthians 3:18, 13:14; Hebrews 9:14; also compare 1 Peter 3:18 with Acts 2:24).
You are right, however, in distinguishing between the blasphemy against the Holy Spirit and all other blasphemy. Our Lord Himself made such a differentiation. In Matthew 12:31-32, Jesus said, “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.”
While our Lord’s teaching in these verses is most solemn, it does have a positive side to it. It teaches us that “all manner of sin and blasphemy” is pardonable, with the lone exception of “the blasphemy against the Holy Spirit.” Even blasphemy against the Father and the Son is forgivable, as Jesus makes clear by saying: “All manner of…blasphemy shall be forgiven unto men…And whosoever speaketh a word against the Son of man, it shall be forgiven him.” It is only the sin of blaspheming the Holy Spirit that is singled out by our Lord as unpardonable
What is this unpardonable sin? Who can commit it and how is it committed? In Matthew 12, our Lord’s words on this solemn subject were prompted by the Pharisees attributing of Christ’s power to “cast out demons” to “Beelzebub the prince of demons” (Matthew 12:24). In response to such hardness of heart, Jesus points out the absurdity of “Satan casting out Satan” and issues to these hard-hearted Pharisees a warning about speaking such blasphemy against the Holy Spirit (Matthew 12:25-32). It would appear, therefore, that the only ones in danger of perpetrating this lone unpardonable sin are hard-hearted unbelievers, like the Pharisees, whose hearts have so hardened that they not only deplore Christ, but are also disparaging of and desensitized to the Holy Spirit of God.
The Holy Spirit is God’s final call to lost humanity. In the past, God sent His prophets to man, but man persecuted them. Then, God sent His Son, but man crucified Christ. Finally, God has sent the Holy Spirit into the world. The Holy Spirit is God’s final invitation to fallen man. If man hardens his heart to the Spirit’s invitation to come to Christ there remains no further hope of salvation. The sinner has committed the unforgivable!
Each time the sinner says “No” to the Holy Spirit makes it that much easier to say “No” the next time. The sinner’s heart becomes harder and harder, eventually becoming so calloused that the sinner no longer feels the Spirit’s conviction. When the heart becomes calloused, the Spirit ceases to strive with the sinner (see: Genesis 6:3 and Romans 1:24, 26, 28) and the sinner crosses God’s deadline. All hope is lost. The unpardonable sin is committed.
In Ephesians 6:11, the Apostle Paul warns us about “the wiles of the devil.” Perhaps, the devil’s greatest “wile” is “wait awhile.” The devil never walks up to unbelievers and asks them to sign over their immortal souls. He’s much too subtle for that. Instead, he simply seeks to persuade them to procrastinate when it comes to making their decision for Christ. He assures them that there is no rush; they have plenty of time to respond to the Spirit’s invitation. Once deceived by the devil into believing that there is no urgency in the Spirit’s conviction, the sinner is well on his way to a calloused heart and the perpetration of the unpardonable sin.
This soul stealing strategy of Satan is most effective in our day and time, thanks to the widespread misconception that sinners can be saved anytime they take a notion to. The truth, however, is that salvation is not something to be had whenever the sinner says. Instead, as the Scripture makes clear, salvation is something that is only possible whenever God says.
The sinner can only be saved when God calls, not whenever the sinner chooses. This is why the Bible repeatedly warns us not to harden our hearts on the day we hear God’s voice (Psalm 95:7-8; Hebrews 3:7-8, 15). Since sinners can only come to Christ when they are under the conviction of the Holy Spirit; that is, when they are hearing God’s voice, they must not harden their hearts to the Spirit’s conviction. If they do, they have no guarantee of ever hearing God’s voice again. If sinners harden their hearts to the voice of God today, their coming to Christ could be an impossibility tomorrow.
Obviously, the unpardonable sin—blasphemy against the Holy Spirit—can only be committed by someone who is lost. It cannot be committed by a Christian, one who has responded to the Spirit’s conviction by coming to faith in Christ. Thus, no Christian need ever fear that he/she has committed an unpardonable sin.
Now, at last, we come to your question about blasphemy. Is it impossible for a Christian to commit the sin of blasphemy? In 1 Timothy 1:19-20, the Apostle Paul talks about two men who had “made shipwreck” of their faith. According to Paul, these two men, Hymenaeus and Alexander, had been “delivered unto Satan” so that they might “learn not to blaspheme.”
Apparently, Hymenaeus and Alexander were Christians, since they could not have made shipwreck of something; namely, faith, that they never possessed in the first place. Also, they are not portrayed by Paul as being handed over to Satan for condemnation, but for chastisement—“that they may learn not to blaspheme.” All things considered, it appears that the Scripture affords us in this instance a clear example of a couple of Christians who were guilty of blasphemy.