I don't have a solid view on it myself. I perused the thread perhaps too casually, but no one (that I noticed) raised a question that goes round in my mind. It seems to me that if I could understand the passage before "sin unto death" is mentioned, I'd have a better chance of understanding it.
1 John 5:14 And this is the confidence that we have in him, that, if we ask any thing according to his will, he heareth us:
15 And if we know that he hear us, whatsoever we ask, we know that we have the petitions that we desired of him.
16 If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it.
17 All unrighteousness is sin: and there is a sin not unto death.
I write tentatively and not dogmatically. If I see someone sinning (and I am willing to count non-Christians among my brothers since we all have Adam as a father), I can ask for that sin to be forgiven, and it will be. Do I have that right? I think I do if I have the right relationship with God.
John 20:23 Whose soever sins ye remit, they are remitted unto them; and whose soever sins ye retain, they are retained.
If we love someone else, it creates a tie. Bits of our identity are exchanged. I would not be complete in Heaven if that person was missing.
Compare this to why God gave the Land of Promise to Israel -- not on their own account!
Deuteronomy 9:5 Not for thy righteousness, or for the uprightness of thine heart, dost thou go to possess their land: but for the wickedness of these nations the Lord thy God doth drive them out from before thee, and that he may perform the word which the Lord sware unto thy fathers, Abraham, Isaac, and Jacob.
6 Understand therefore, that the Lord thy God giveth thee not this good land to possess it for thy righteousness; for thou art a stiffnecked people.
It compares to something Paul wrote as well about the "law of Christ."
Galatians 6:2 Bear ye one another's burdens, and so fulfil the law of Christ.
The mature Christian is to help bear the burdens of the "babes in Christ" -- it seems to me. Eventually that relationship is to stop and does when the "babe" grows up.
Galatians 6:4 But let every man prove his own work, and then shall he have rejoicing in himself alone, and not in another.
5 For every man shall bear his own burden.
Are their limits to the burdens a Christian should bear for others? I think 1 John 5:16 says yes. If you could lay down your life to save someone else and it worked, that would be acceptable, wouldn't it?
John 15:13 Greater love hath no man than this, that a man lay down his life for his friends.
It becomes unacceptable with a certain type of sin. Suppose I had a friend on drugs who had no job, no place to live and with next to nothing to look forward to. I offer him a place to live so he can wash up to get a job, so he can use my phone and address. What should I do if he abuses my kindness? What if he steals from me to buy drugs? What if he gets a job and then blows his first paycheck on drugs? I don't think it would help to keep subsidizing him. He's on a course to death. I should not jeopardize my own welfare and waste resources on him when they could do good elsewhere.
There are four types of relationships: Returning good for good and returning evil for evil are easy to understand. Returning good for evil usually works by breaking down the other person's distrust of others -- if he believes love is not really possible, you may convince him it is. You have helped bring him to his senses and back to the realization he is a child of God. The fourth type is horrifying: Someone receives good from you and returns evil. This person knows what good is. He knows you are a good person, and he sees you as a victim.
Not everyone can sin against the Holy Spirit. You have to know what it is first. The religious leaders Jesus criticized knew what the Holy Spirit was, and they knew Jesus had it. They knew because they once had the Spirit themselves; but looking around, they were tempted to betray the Holy Spirit for money, influence and maybe sex. They treated ordinary Jews like sheep to be fleeced. It was easy to do until Jesus came along, making them feel threatened. When they saw the good in Jesus and called it evil, they were betraying themselves as well as sinning against the Spirit. This can reach a point of no return where someone is spiritually insane.
You are wasting your time praying for people who return evil for good; and you are endangering yourself spiritually by tying yourself to them. They can be like vampires sucking the Life and Light out of you. I cannot tell anyone where the line gets drawn; but I've reached it a few times -- I simply gave up completely on some people. I loved them once, but I cut those ties -- if they choose the path to hell, I will not pray for them, I will not be dragged down with them by tying my fate to theirs. Maybe Paul would call it delivering them to Satan.
Does it mean those people are going to be eternally damned? I don't think so. If I stop enabling them, that might make them shape up. I don't count on it; but if enough people stopped enabling them, they would have a better chance at seeing the errors of their ways than if they continue to view good people as prey. What it may mean is that no one will bear their burdens for them -- not me, not Jesus, not anyone. They will pay a penalty then.
Jesus tells an interesting story about a man who was forgiven of his debts. He finds out later his master undoes that "forgiveness" and demands payment in full. This wicked man liked "goodness" when he was receiving it, but he had no pity for others and resisted "goodness" when it would "cost" him something. I think this man committed the sin without pardon -- no pardon -- he will have to pay himself.
Matthew 18:32 Then his lord, after that he had called him, said unto him, O thou wicked servant, I forgave thee all that debt, because thou desiredst me:
33 Shouldest not thou also have had compassion on thy fellowservant, even as I had pity on thee?
34 And his lord was wroth, and delivered him to the tormentors, till he should pay all that was due unto him.
35 So likewise shall my heavenly Father do also unto you, if ye from your hearts forgive not every one his brother their trespasses.
Shakespeare wrote: "He that dies pays all debts." I think there is a similar Jewish saying. Body and soul may be destroyed, but the spirit can still be saved.