If anyone sees his brother committing a sin not leading to death, he shall ask, and God will give him life—to those who commit sins that do not lead to death. There is sin that leads to death; I do not say that one should pray for that. 1 John 5:16

εάν    τις         ίδη         τον αδελφόν αυτού αμαρτάνοντα αμαρτίαν    μη
If     anyone    should see        brother his   sinning     a sin       not

προς θάνατον αιτήσει       και δώσει            αυτώ    ζωήν τοις
to   death,  he shall ask, and he shall give to him    life, to the ones

αμαρτάνουσι μη προς θάνατον έστιν   αμαρτία προς θάνατον ου
sinning     not to  death. There is sin     to   death;  not

περί       εκείνης λέγω     ίνα  ερωτήση
concerning that    do I say that he should ask.

There are three keys in understanding this verse:

1. These verses are directed at the believer – one that has been born again.
2. There is a sin that is not unto death, and there is sin that is unto death: the sin unto death is that of denying Christ. This the antichrists do – as John alluded to in chapter 2:19, but is what in essence all unbelievers do. The sin that is not unto death is what John speaks of in chapter 1. John demonstrates that Jesus is “just and righteousness” and therefore will forgive the repentant heart.
3. In this verse John is speaking of a person who approaches God in an intercessory manner – the key to this verse is that a person can and should ask of God, but in all requests there are certain boundaries – no person can ask God to do what is not in His will. That is, our requests must respect God’s righteousness and holiness. He will show perfect justice, we cannot ask differently.

What does prayer mean here?

The word translated “pray” in some Bible versions is more rightly translated “request” or “ask” (erotese) as seen in the verse, shown here. The form of this word appears 63 times in the Bible. A human cannot be an intercessor for a person who has sinned unto death, that is refuses to believe on Jesus Christ. In some sense, the action entertains the notion of forgiving sin, but only God can forgive sin, through the shedding of the blood of Jesus. The “ask” or “request” here is one of a sinner humbly asking the Holy God for forgiveness. Humans can do no other. Martha made the mistake in assuming that Jesus was “asking” his Father for Lazarus to rise , forgetting that Jesus, the Son of God, was not inferior to God, and therefore does not “ask” - Jesus prays – in a worshipful manner (i.e. Mark 14:39) – hence a different Greek word is used.

The Apostle John is showing that asking God that a so called fellow believer does not sin unto death is a humble act, which accords with God’s will. To intercede where no authority has been given – i.e. for a brother who sins unto death (denies Christ), is one of presumption, and well outside the scope of a Christian. It is asking for more mercy than God has shown, and therefore one of presumption. And in any case as JFB points out a brother sinning unto death demonstrates that he has not ever been a true brother, and therefore, the requestor is essentially asking that God does away with His righteousness and judges in a manner that is un-righteous. Such a requestor is essentially telling God not to judge sin in this person, although all sin requires death as the punishment – the sinful nature needs to be done away, the person reborn with a spiritual nature. That one can appear to be a brother, but is not, is dealt; see 1 John chapter 2.

"Little children, it is the last hour; and as you have heard that the Antichrist is coming, even now many antichrists have come, by which we know that it is the last hour. They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us." (1 John 2:18-19)

The verses accord to what Paul told Timothy:

"Take heed to yourself and to the doctrine. Continue in them, for in doing this you will save both yourself and those who hear you." (1 Timothy 4:16).