I have heard many people give various meaning to 1 john 5 vs 16 and 17. Some of them, hilarious. Some others, sorrowful.

But let me hear from you. What really is the Sin unto death. The answer is very simple. Share yours before i share mine.

ublight:
■ I have heard many people give various meaning to 1 john 5 vs 16 and 17. Some of them, hilarious. Some others, sorrowful. But let me hear from you. What really is the Sin unto death. The answer is very simple. Share yours before i share mine.

The sin unto eternal death aka Eternal Damnation aka the Second Death spoken of by John in this context is the sin of blasphemy against the Holy Spirit.

13 I write this letter to you who believe in the Son of God. I write so that you will know that you have eternal life now.
14 We can come to God with no doubts. This means that when we ask God for things (and those things agree with what God wants for us), God cares about what we say.
15 He listens to us every time we ask him. So we know that he gives us whatever we ask from him.
16 Suppose you see your fellow believer sinning (sin that does not lead to eternal death). You should pray for them. Then God will give them life. I am talking about people whose sin does not lead to eternal death. There is sin that leads to death. I don’t mean that you should pray about that kind of sin.
17 Doing wrong is always sin. But there is sin that does not lead to eternal death.
18 We know that those who have been made God’s children do not continue to sin. The Son of God keeps them safe. The Evil One cannot hurt them.
[b]19 We know that we belong to God, but the Evil One controls the whole world.
20 And we know that the Son of God has come and has given us understanding. So now we can know the one who is true, and we live in that true God. We are in his Son, Jesus Christ. He is the true God, and he is eternal life.
21 So, dear children, keep yourselves away from false gods. - 1 John 5 vs 13 - 21

The consequence for blasphemy against the Holy Spirit is automatic and no amount of praying for such a person condemned for that sin can help or change the ruling against such a person.