There are a few passages where the Bible speaks of a sin against the Holy Spirit who could not be forgiven. Many Christians wonder what is this sin which cannot be forgiven, and especially why can it not be forgiven if God forgave all our past, present and future sins on the cross.
When John speaks of a sin that leads to death, it is indeed sin against the Holy Spirit.

1 John 5: 16-17  If anyone sees his brother sinning a sin which does not lead to death, he will ask, and He will give him life for those who commit sin not leading to death. There is sin leading to death. I do not say that he should pray about that. All unrighteousness is sin, and there is sin not leading to death.
John says that it is not even necessary to pray for this sin that leads to death. This sin is the sin, or the blasphemy, against the Holy Spirit whom Jesus speaks about in:

Matthew 12: 32  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.

Mark 3: 29  but he who blasphemes against the Holy Spirit never has forgiveness, but is subject to eternal condemnation”

Luke 12: 10 “And anyone who speaks a word against the Son of Man, it will be forgiven him; but to him who blasphemes against the Holy Spirit, it will not be forgiven.

It is clear in fact that the sin against the holy Spirit which the Bible speaks about in various passages of the New Covenant is irrevocable, there is no turning back.

But why?

Sinning against the Holy Spirit is equivalent to give back one’s salvation to God.
We must first consider that salvation in Christ is unchangeable, once acquired it is impossible to lose it. We are saved once and for all, because Christ died once on the cross and won’t return onto it. We will not get now into those details because this is another topic that you can find in another one of our "Quick Teachings."
Only a born again person can sin against the Holy Spirit.
What for?

Simply because giving back his salvation God requires to first have received it.
In practice, for a person who is saved, sinning against the Holy Spirit would be equivalent to say to God: ' I don't want any more that salvation in Christ in my life. I prefer to follow the devil because I consider that it is more powerful and more glorious than You ».
In this case God respects our choice, because He loves us.
We are not talking here about a person who would say such words to God out of anger, or as a result of a depression or something else, but about a long pondered decision taken in full knowledge of its outcome.

Of course such decisions are rare among Christians, but nonetheless exist unfortunately.

Why is the sin against the Holy Spirit irrevocable?

Why can it not be forgiven?

It is not because God would be angry and would reject anyone who knowingly abandoned Him. The reason is totaly different.
If a person who is saved rejects God for his life because he considers that satan is much better, it's a deliberate choice that he is making. God gives us this choice because He wants us free to make our own decisions.
Because He loves us, He wants us free of our choice. In this sense, each Christian has the choice to live in the grace for eternity, but he also has the choice to give this salvation back to God by telling Him so, in a thoughtful way, and having properly assessed all the consequences. He will therefore give back his salvation in Jésus Christ.
In this case, it will not possible to that person to get his salvation back if he changes his mind later on.
Not that God would be mad at this person, on the contrary, but simply because he has already received once the salvation in Christ at the cross. The sacrifice of Jesus on the cross is not an access that would allow us to go back and forth into the Kingdom of God. This sacrifice, like all the ones of the Old Covenant, is unique, in the sense that we can be benefit from it only once.

For a person who has sinned against the Holy Spirit to be able somehow to have his salvation restored, Jesus would have to go back on the cross another time. This fact alone shows that on one hand, the salvation in Christ cannot be lost because one would have committed such or such 'important' sin, other than sinning against the Holy Spirit. And on another hand, it also shows us that this same salvation is granted once and for all, the same way that Christ died on the cross once and for all.

Numerous passages confirm this:
Hebrews 7: 27  who does not need daily, as those high priests, to offer up sacrifices, first for His own sins and then for the people’s, for this He did once for all when He offered up Himself.

Hebrews 9: 12  Not with the blood of goats and calves, but with His own blood He entered the Most Holy Place once for all, having obtained eternal redemption.

Hebrews 10: 10  By that will we have been sanctified through the offering of the body of Jesus Christ once for all.

Jude 1: 3  Beloved, while I was very diligent to write to you concerning our common salvation, I found it necessary to write to you exhorting you to contend earnestly for the faith which was once for all delivered to the saints.

Romans 6: 10  For the death that He died, He died to sin once for all; but the life that He lives, He lives to God.

We can benefit only once from the salvation in Christ, because Jesus died once at the cross. This shows, among other things, that salvation cannot be lost and be restored, but also that if a person cancels willingly his salvation (sinning against the Holy Spirit), it will unfortunately be impossible to be restored, for the reasons mentioned above. .
Sinning against the Holy Spirit means canceling purely and simply his salvation, expressing the fact to be erased from the Book of Life in heaven. This action is irrevocable, that's why this sin cannot be forgiven. Not that God would not forgive it, but simply because people alone would condemn themselves.

This is what Jesus explained here:
Matthew 12: 24-31  Now when the Pharisees heard it they said, “This fellow does not cast out demons except by Beelzebub,[a] the ruler of the demons.” But Jesus knew their thoughts, and said to them: “Every kingdom divided against itself is brought to desolation, and every city or house divided against itself will not stand. If Satan casts out Satan, he is divided against himself. How then will his kingdom stand? And if I cast out demons by Beelzebub, by whom do your sons cast them out? Therefore they shall be your judges. But if I cast out demons by the Spirit of God, surely the kingdom of God has come upon you. Or how can one enter a strong man’s house and plunder his goods, unless he first binds the strong man? And then he will plunder his house. He who is not with Me is against Me, and he who does not gather with Me scatters abroad.“Therefore I say to you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven men.

The Pharisees knew that Jesus was the Son of God, they knew very well that the accomplished miracles came to pass with the power of God.
But they knowingly preferred to assign and proclaim that the power of God manifested through Jesus, was in fact a satanic power.
In other words, they attributed the glory and the power of God to satan. They considered that satan was worthier to receive the power and the glory, instead of God.
Now, we can understand the sin against the Holy Spirit.
The sin against the Holy Spirit is irrevocable, simply because you can have access to the salvation in Christ only once.
As we said, it would require Jesus to return to die one more time on the cross so that a person who would have rejected his salvation, could go through the new birth again.

This is why the Bible also says:
Hebrews 6: 4-6  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.

This shows on one hand, that it is impossible to lose our salvation because we would have committed some particular sins, and to get it back after repentance. It is therefore impossible to lose our salvation other than by the fact of sinning against the Holy Spirit.
The author of the Epistle to the Hebrews shows us then this first fact, but also that if a person would willingly give his salvation back to God, fully knowing the consequences of doing so, rejecting it and preferring to give the power and the glory to satan, it would be impossible for him to backtrack.

Bye for now.