“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” (1 John 5:16).
John is writing to believers, and the death he’s talking about is physical death. That is, if a believer commits that sin and persists in it, there will come a day when God will reach down and take that child of His out of this world by death. In other words, death would be the judgment of God upon him, removing him from the scene and place of witness.
How do I know whether a sin is unto death? Is it a certain specific sin? No. For different men it might be different things, so when you see a brother sin a sin that you don’t know is unto death, you shall not sit in judgment on him, you should not criticize him for it, but you should go to God in prayer and pray that it might not be a sin unto death and that the Lord might keep him in the world. That’s the reason why the sin is not mentioned.
As God’s children, He can’t let us just go on and on in sin because He must judge us. And when we will not be disciplined and will not turn from it, then He will finally reach down and call us home. For you it may be one thing, for me it may be something else. But when you see any child of God commit a sin not unto death, you should pray that the Lord would give that child of His another opportunity.