Is there an "unpardonable sin" that cannot be repented of?

Four verses to consider: I John 5:16-17; Matthew 12:31; Mark 3:28; Luke 12:10

First, I John 5:16-17
16If 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. 17All unrighteousness is sin, and there is sin not leading to death.

Some say the "sin unto death" in these verses is the same as the "blasphemy against the Spirit" in Matthew 12:31. I don't believe it is.

The topic under discussion here is Prayer. For what may a Christian pray with assurance of being heard and answered? The answer is in verses 14 and 15:

14Now this is the confidence that we have in Him, that if we ask anything according to His will, He hears us. 15And if we know that He hears us, whatever we ask, we know that we have the petitions that we have asked of Him."

So, when it comes to praying for "life" (spiritual life or the forgiveness of sins) of behalf of a brother, God's will must be respected here as in all other matters.

Earlier in this book, I John 1:9: 9If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness.

Lets now read the other verses:

Matthew 12:31
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. 32Anyone 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 3:28
28"Assuredly, I say to you, all sins will be forgiven the sons of men, and whatever blasphemies they may utter; 29but he who blasphemes against the Holy Spirit never has forgiveness, but is subject to eternal condemnation"-30because they said, "He has an unclean spirit."

Luke 12:10
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."

I believe that Mark 3:30 explains it: because they said, "He has an unclean spirit." If anyone ascribes Christ's works to demoniac influence, he is blaspheming against the Holy Spirit. These works were performed by the power of God.

Blaspheming Against the Holy Spirit

Matthew 12:31-32 Jesus tells of an unforgivable sin.
Jesus had just healed a demon-possessed man 22-23
They attributed His power to Beelzebub (Satan) 24
He illustrates the absurdity of their charge 25-28

Mark 3:28-30
What is Blasphemy of the Holy Spirit?
It is ascribing the work of the Holy Spirit to Satan

Luke 12:10
Why is this sin unforgivable?
When one concludes that Jesus' power was of the devil.

He has rejected the evidence, which produces faith in Jesus
He has rejected the efforts by God to save those in sin

When one is willing to believe that Jesus was in league
with the devil, saving faith is not possible.