Question: Could you please explain the sin unto death in 1st John 5:16 to me?
Response: I will certainly try. Let me say by way of preface that I have a detailed treatment of this subject in the Hamartiology study under "Apostasy and the Sin unto Death".
Here's my translation of 1st John 5:16:
If anyone sees his brother sinning sin (i.e., involved in sin) which is not unto death (i.e., is not "deadly"), let him ask [forgiveness on his brother's behalf], and life will be given to him (i.e., forgiveness and deliverance), that is, in those cases where those sinning are not [sinning] unto death (i.e., engaged in "deadly" sin).
It is pretty clear from the context that we are to be concerned for the welfare of our fellow believers as regards their spiritual lives, not only from the positive point of view (i.e., ministering to them to promote their spiritual health and growth), but also when it comes to helping them break free from the negatives of life. That is to say, if a fellow believer is known to us to be committing sin in such an obvious way that it has come to our attention, that person is clearly in some serious spiritual trouble. We all sin - John is clear about that in chapter one - but, even though sin is sin, from the standpoint of growth, momentum and even spiritual safety, there is a difference between occasional lapses quickly repented of and confessed to God, and a broad pattern of deliberate and arrogant sin. For the Bible does clearly make such a distinction: if we are involved in a pattern of wrong behavior that is clearly "in God's face", so to speak, such lack of fear and respect for Him is part of the problem:
Who can discern his errors? Acquit me of hidden faults. Also keep back Thy servant from presumptuous sins; Let them not rule over me. Then I shall be blameless, and I shall be acquitted of great transgression.
Psalm 19:11-14 NASB
I would venture to say that many believers at some time or the other have fallen into a pattern of this sort in respect to some area of personal weakness (we all have them, after all), and it is not only legitimate, but important that when such behavior becomes obvious that it is noticeable to other believers, they should intervene in prayer, and occasionally even in a more personal way (but with great care: cf. Jas.5:19-20; Jude 22-23).
It is important to note in this regard that censure, self-righteousness and verbal condemnation is absolutely the wrong attitude as is clear from the context of 1st John 5:16 above. Even though our brothers and sisters may be way off the mark, the right attitude for those who would help them is one of compassion and a desire to assist get them in getting back on the right track for the sake of the whole Body of Christ and for own their personal spiritual well-being and safety as well. There are many instance when it will not be possible to do more than to pray for such persons, pray for the forgiveness of their sins (for if they are "high-handedly" pursuing a wrong course, they are not in confession of such sins to their Father, of that you can be relatively sure - contrast 1Jn.5:18 where we are told that the true child of God does not persist in sin), and pray for God to bring them back to the right path.
However, there are also times when no personal intervention and no amount of personal prayer will help - John does not obligate us to pray for such situations. There are clearly cases - all too many in our own day - when the chances of a person escaping the devil's snare are nigh on impossible (sometimes all we can do for such people is to state/live the truth and hope: 2Tim.2:26). Why is this? Because once someone has rejected God long enough and deliberately enough, the conscience becomes seared, the heart hardened, and the will set against Him - this is one and the same with the death of faith, and if faith is dead, there is nothing whereby the person can be saved, because all salvation is through faith in Christ. Faith is a living plant that needs to be nourished and needs to grow; faith needs to be faithful. One cannot say "Lord, Lord", and "not do what I [Jesus] say" forever and ad nauseum without consequences (Lk.6:46-49). It is impossible to really believe in Jesus and be His believing follower, and at the same time give oneself over whole-heartedly to sin without damaging and eventually killing one's faith - not because believers can't sin, but because we who truly love Him have a consciousness of rebelling against Him when we sin, and because the prodding of the Spirit will always lead us to repentance and restoration through confession, or else to greater reaction and rebellion if we do not heed the calls to return to Him. In the latter case, this can, when taken to the extreme, bring about the complete and total death of faith, wherein the person who had once professed Christ becomes so hardened against Him in the pursuit of unconfessed and self-justified sinning that such a person no longer has any respect for Him, any concern for what He wants, any intention of following Him anywhere, or any shame or regret for their rebellion - in short, such a person is no longer a believer because any genuine love, concern, faith in or faithfulness to our Lord has long since been snuffed out by their own actions (this is also known as "apostasy": on which see in particular Peter's Epistles #21: "Perseverance of Faith" and Peter's Epistles #26: "Reactions to Personal Tribulation").
When we see someone already in this tragic condition, where the seed planted has sprouted but died, there is no point in further prayer, because that person is simple no longer a believer in Jesus Christ (and close inspection/interrogation will prove this point well enough); they have returned to their own vomit, and the end is worse than the beginning so that it truly would have been better for them never to have professed Christ in the first place (2Pet.3:22).
Jesus told us that whatever we bind on the earth will be bound in heaven and whatever we loose will be loosed in heaven (Matt.18:18). I take this to mean, among other things, that we can pray for the sins of our fellow believers and have confidence that God will take their case in hand, forgiving them (where they have been too blind to turn to Him themselves), and working for their restoration. On the other hand, there are also situations where divine intervention and restraint is needed, and, while this covers a lot of territory (the suppression of all sorts of evil), it can also mean the prayer to hinder the continued sin of wayward believers (this may involve much pain, as when Paul "handed over" a Corinthian in such a state to the devil: 1Cor.5:4-5). Once someone has passed beyond the point of any possible responsiveness, however, there is nothing much left to do besides committing that person to the grace of God and moving on. In such cases, I have always taken comfort in the fact that "nothing is impossible with God", but I do need to stress here that in this verse John is relieving us of the responsibility of excessive worry and concern in the case of individuals who have, of their own free will, placed themselves beyond the pale of grace (as far as we can discern it or are in a position to affect it, even by our prayers). It is also important to note here that John's whole thrust in this passage is to assure his listeners that all not sin is "unto death" - that is to say, we should not assume that, because we see a fellow believer involved in sin, we should therefore immediately wash our hands of him/her because we feel that they are now "lost". Scripture is clear that great circumspection is necessary in such cases (Jude 22-23), but that does not mean we should "cut loose" those whose sinfulness we notice may be something less than complete and utter apostasy and rejection of Christ.
We all need God's mercy, and can all stand to benefit from the intervening prayers of our fellow believers from time to time, but we also have to be realistic that not everyone is "of the faith" (2Thes.3:2). And there are certain behavior patterns embraced by an individual with free will which almost by definition proclaim the complete rejection of any semblance of Christianity (becoming a drug pusher or member of a criminal organization, for example). In such instances, John relieves us here of the burden of intervention.
I take great comfort and encouragement in this verse from the biblical fact that - apart from the most clear-cut cases of faith-denying sinfulness - we have the opportunity to help and intervene for our fellow followers of Christ through prayer whenever we see them in trouble (and also to have them do the same for us). Such is great mercy of our God!
You might also want to see the following links:
The Great Apostasy
The Danger of Apostasy
The False Doctrine of Absolute Eternal Security I.
The False Doctrine of Absolute Eternal Security II.
The unpardonable sin and blasphemy against the Holy Spirit.
Have I committed the unforgivable sin?
Yours in the Lord of mercy.