The Dark Region Of Despair -1 John 5:16

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. - 1 John 5:16

A most curious verse.  Doesn’t all sin lead to death? What sin are we talking about here? Are there sinners we should pray for and sinners we should abstain from praying for?
True, all sins leads to death - “For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord.” - Romans 6:23. Repentance and belief in Jesus Christ rescue us from the sin that does not lead to death, but what of the sin that John speaks about?
Obviously, John is speaking of the “unpardonable sin.”  “Therefore I say to you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven men. 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.” men. 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.” - Matthew 12:31-32. Clearly, even Jesus saw at least one unpardonable sin. What is the defining difference between a sin that does not lead to death and the sin which does lead to death? Charles H. Spurgeon addressed this quandary perhaps better than anyone else - “He who has committed the sin which is unto death have no desire for forgiveness, he will never repent, he will never seek faith in Christ but he will continue hardened and unbelieving; he will henceforth never be the subject of holy influences, for he has crossed over into that dark region of despair where hope and mercy never come.”  The sin of unbelief is the sin which defies forgiveness because it defies belief in Jesus Christ! The Holy Spirit is the witness to the glory of Christ. To refuse to believe in His witness is blasphemy and as Jesus tells us it will not be forgiven the unbeliever in this age or the next. Prayers offered for the soul of an unbeliever who has consistently denied the glory of Christ are prayers which will not help that individual since his unbelief accompanied him throughout his life and across the threshold into death. But how can we know beyond a shadow of doubt who has committed this unpardonable sin? The fact is only God can know for certain - “But the Lord said to Samuel, “Do not look at his appearance or at his physical stature, because I have refused him. For the Lord does not see as man sees; for man looks at the outward appearance, but the Lord looks at the heart.”
 - 1 Samuel 16:7.
Since it is impossible for us to discern the heart within our fellow man, or “brother” as the title verse suggests, we cannot know for certain. We should err on the side of grace and pray without ceasing for all mankind. You see, at the end of the verse John merely says that we are not obliged to pray about it, but he doesn’t say we shouldn’t. May we always pray that the glory of Christ is illuminated in the heart of every man, woman, and child. Let there be no unbelievers among us.