Jesus Christ spoke of a sin, blasphemy against the Holy Spirit, that will not be forgiven.

Mat 12:31  “Therefore I say to you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven men.
Mat 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.

Mar 3:28  “Assuredly, I say to you, all sins will be forgiven the sons of men, and whatever blasphemies they may utter;
Mar 3:29  but he who blasphemes against the Holy Spirit never has forgiveness, but is subject to eternal condemnation“—
Mar 3:30  because they said, “He has an unclean spirit.”

Luk 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.

Twice in Hebrews, the author also speaks of a sin that will not be forgiven.

Heb 6:4  For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit,
Heb 6:5  and have tasted the good word of God and the powers of the age to come,
Heb 6:6  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.

Heb 10:26  For if we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins,
Heb 10:27  but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries.

The warning of an unpardonable sin—a sin that will not be forgiven—was given to two different groups.  Jesus warned the religious leaders, who were unbelievers.  The author of Hebrews warned believers.

Are these sins the same?  And what are they exactly?  Let’s take a careful at what the Bible says.

Jesus cast out demons from many people by the power of the Holy Spirit.  The religious leaders knew this, yet they accused Jesus of being demon possessed and casting out demons by the devil.  The Pharisees knowingly and purposefully rejected God.  Jesus’s statement about blasphemy against the Spirit was a warning to them that they were in danger of losing out on eternal life.  For a more detailed discussion on this see, “What is blasphemy against the Holy Spirit?”

Hebrews 6:4-6 gives a warning to Christians, those who are enlightened, who have received the Holy Spirit, who have tasted the heavenly gift, that if they fall away, it is impossible for them to repent.  Hebrews 10:26-27 explains further that the falling away is a wilful rejection.  In other words, if someone understands and accepts the truth, repents, receives the Holy Spirit, and then later knowingly and purposefully rejects God, they cannot repent again.

How does such a thing happen?  How can it become impossible for someone to repent?  In our Christian lives, we sin.  But we come before our heavenly Father and ask for forgiveness. 

1Jn 1:9  If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness.

The problem is with the sinner.  God is telling us that we can harden our hearts against God to the point of no return where we no longer want to repent and can never ever want to repent.  A man refusing to repent is like a man walking down a mountain side that’s getting steeper and steeper.  Eventually he slips and falls down the mountain, unable to save himself.  As another analogy, think of a stick being held upright.  It has flexibility to bend from the upright position, representing sin, and return, representing repentance.  But if the stick is bent too far it snaps in two, representing the impossibility of repentance.

In both situations, the warning Jesus gave the unbelieving religious leaders, and the warning given to believers in Hebrews, the warning is about a knowing and purposeful rejection of God.  God is warning us of a reality.  If we knowingly and purposefully reject God for too long, there is a point of no return where we cannot change, and our fate is eternally sealed.

It is natural to ask the question, “Have I committed the unpardonable sin?”  The answer is, if you still want to repent of your sins then you haven’t.

Conclusion

The unpardonable sin—a sin that will not be forgiven—is possible for both unbelievers and believers alike to commit.  It is a knowing and purposeful rejection of God, to the point of no return where the sinner is incapable of desiring repentance.