Blasphemy against the Holy Ghost.
“For this reason I say unto you, Every sin and injurious speaking shall be forgiven to men, but speaking injuriously of the Spirit shall not be forgiven to men. And whosoever shall have spoken a word against the Son of man, it shall be forgiven him; but whosoever shall speak against the Holy Spirit, it shall not be forgiven him, neither in this age nor in the coming one.” (Matthew 12:30-31)
Who can commit blasphemy against the Holy Ghost? Strictly speaking, these verse apply to those to whom the Lord was speaking; the class of apostate Jewish leaders who had witnessed first-hand the ministry of the Holy Spirit. The Lord is speaking to a class of persons, later called “this generation” (see note). However, He does say “whosoever” bringing forward the consequences for individuals who find themselves in that class. Hebrews makes it clear that the testimony of the Holy Spirit continued after the Lord was on earth; “so great salvation; which at the first began to be spoken by the Lord, and was confirmed unto us by them that heard him; God also bearing them witness, both with signs and wonders, and with divers miracles, and gifts of the Holy Ghost, according to his own will…” (Heb. 2:3-4). It is not even possible, technically speaking, to blaspheme the Holy Ghost today, as the Lord and His apostles are no longer here.
What about today? However, there is a solemn passage in Hebrews which applies this same principle to Christianity. “He that despised Moses’ law died without mercy under two or three witnesses: of how much sorer punishment suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God …and hath done despite unto the Spirit of grace” (Heb. 10:28, 29). In this sense, it is possible for a person today to commit despite to the Spirit of grace. Who can do that? Only an apostate. An apostate is one who once had a profession of Christianity and partook of its blessings, but then fell away from “the faith”, never truly having possessed “saving faith”. Merely rejecting the gospel does not constitute blasphemy against the Holy Ghost. There were 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” and yet remained unconverted. If they were to reject that testimony of the Spirit, “it is impossible… to renew them again unto repentance.” Incorrect use has been made of this verse to teach that the believer’s security is conditional, and that it is possible, once saved, to be lost again. This is false, for it denies many other scriptures, such as John 10:27-29. Only an unbeliever could really say with open-eyed, deliberate hatred toward God, that the Holy Spirit is demonic.
Governmental Forgiveness vs. Eternal Forgiveness. We must bear in mind that there are several aspects of forgiveness. Eternal forgiveness is not spoken of prior to the cross. It is most often governmental forgiveness that is taken up. Governmental forgiveness is divine exemption from the governmental consequences of our sins in this life. A person may remain under the government of God until death, perhaps due to an unforgiving spirit, but then go to heaven (Matthew 6:14-15). Here the Lord is speaking to the leaders of apostate Israel. Israel will never again be forgiven this blasphemy and restored to Jehovah en masse. To be clear, they will be nationally restored (Rom. 11:26), but it will be through a remnant. You can see that it is governmental forgiveness because the Lord jumps forward to the millennium; “neither in that age which is to come”. Certainly, those who he was speaking to have long since died. But that same moral class will be present at the close of this epoch, and they will not be allowed to enjoy the blessings of the Spirit in the Millennium. They had “tasted” of the Spirit’s grace, and rejected it. And while a remnant will be brought into the full enjoyment of the Spirit in the Kingdom, the apostate ruling class will be cut off in the Great Tribulation judgment. However, in that the individuals within the class of “blasphemers” are apostates, the consequences are eternal; for “it shall not be forgiven him“.
Sin against the Holy Ghost. What about believers? What word does this subject have for our conscience? The Holy Ghost is on earth indwelling the House of God. We are exhorted to walk uprightly in light of this fact “And grieve not the Holy Spirit of God, whereby ye are sealed unto the day of redemption” (Eph. 4:30). One particular evil that has pervaded the Church is clericalism. The clerical principle is a special kind of sin against the Holy Spirit, because it denies the Spirit’s place in the Church, and replaces Him with a false system. The clerical principle states that all true ministry flows from the clergy, and any lay preaching is from the Devil. You can see how this evil is of the same character as what the Jewish leaders were guilty of in Matthew 12:30-31. As Israel spoke injuriously against the Spirit in their dispensation, so Christendom has denied the Spirit in our dispensation. The Church has effectively denied the presence and power of the Holy Spirit on earth. Collectively, the Jewish system came into judgment in 70 A.D., and one day the clerical system of Christendom will come into judgment as well, at the middle of Daniel’s seventieth week. While we wouldn’t call the clerical principle “blasphemy” against the Holy Spirit, we can see that it is “sin” against the Spirit, and certainly the dispensational counterpart to what Israel was guilty of. 
If a believer committed this sin, could they lose their salvation? No. None of the elect will ever commit this sin. We know based on the doctrine of election that every person is either a “vessel of wrath” or a “vessel of mercy”. If you are a vessel of mercy, you were chosen in an eternity past and He gave you the faith to believe when the time came. You were elected. A vessel of wrath is a person that was not elected, but when given the opportunity, hardened their heart (like Pharaoh, read Romans 9:14-24) and thus fitted themselves for judgment, despite God’s long-suffering. Later, God hardened Pharaoh’s heart, hence the phrase “it is impossible.” Since none of the redeemed can or will ever commit this sin, there is no contradiction with 1 John 1:7.
A couple helpful realizations that help with this type of question:
- There are two aspects to the atonement of Christ. Propitiation is the aspect in which Christ died to perfectly satisfy God. In this aspect He died for the whole WORLD. Substitution is the aspect in which Christ died for ME. In this aspect He only paid for the individual sins of those who would be SAVED.
- Whenever we come to two verses that seem to contradict each other, we need to accept both as true, even if we can’t comprehend how they can both coexist. For example; “blasphemy against the Holy Ghost shall not be forgiven unto men” is 100% true and “the blood of Jesus Christ his Son cleanseth us from all sin” is 100% true. What I have started to do is simply accept the Word of God, and then ask him to make it clear to me if it pleases Him. And He usually does.
- Darby, J. N. The Notion of a Clergyman: Dispensationally the Sin against the Holy Ghost. Bible Truth Publishers.