1 John 5:16-17
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.
The Grace New Testament Commentary - 1 John 5:16-17
5:16-17 As everywhere in John's epistle, the words "his brother" refer to a real Christian. If then, such a brother is seen sinning a sin which does not lead to death, Christian love should move one to pray for him. This verse might have been better translated "which does not lead directly (or immediately) to death." God sometimes inflicted death immediately in response to certain sins of Christians. The two obvious examples are Ananias and Sapphira (Acts 5:1-11), and the Christians at Corinth who ate the Lord's Supper with unconfessed sin in their lives (1 Cor 11:27-32). John states that there is sin leading directly to death and that he is not saying (I do not say) that a Christian should pray about that. There is no command to pray for such sin, although also there is no command not to either. In other words, if a Christian suspects that a sin leading directly to death is being committed, he is free to pray for the sinning believer, but without any certainty about the outcome of his prayer. Although there is no guarantee, it is always possible that God may "relent" from His judgment. What may a Christian expect when he prays for cases where the sin is one which does not lead directly to death? He (God) will give life for those who commit sin not leading immediately to death. Since the death in question for the sinning brother is not eternal (John 11:26), there is no reason to take life here as eternal either. Since, however, all sin leads ultimately to physical death, to turn from sin leads to a lengthening of one's physical life.
Charles Bing - Grace, Salvation, and Discipleship - Sin Leading to Death, 1 John 5:16
(Salvation) Interpretation: Some sins will cause a believer to lose salvation and go to hell or prove that they were never really saved.
(Discipleship) Interpretation: Some sins will lead to deadness in a believer's fellowship with God and even premature death.
This passage shows the seriousness of certain sins. John writes about a sin that does and a sin that does not lead to death. One's understanding of the death mentioned will lead one to a (Salvation) or (Discipleship) perspective on this difficult passage.
Our first observation is that the "anyone" must be a Christian since that is the audience to whom John writes concerning the reader's "brother." We also recall that there are different meanings for the word death such as spiritual death, eternal death, spiritual deadness, and physical death. But which is spoken of here?
We can begin by ruling out eternal death because John is writing to genuine Christians and a genuine brother who he assumes are saved and thus, cannot lose their salvation. For the same reason, we rule out death as spiritual separation from God. That leaves the possibilities of spiritual deadness and physical death, or perhaps both since one can lead to the other.
John spoke earlier of passing from death to life and abiding in death in 1 John 3:14. These seem to refer to the lack of spiritual vitality or spiritual deadness for those who do not love their brothers. But there is also biblical evidence of believers who sinned and died physically (Acts 5:1-11: 1 Cor. 11:27-32) for committing serious sins against the church. In 5:16, it could be that John has in mind the serious sins that the heretics were perpetrating in the church at Ephesus to which he wrote (see 1 John 1:8, 10; 2:18-19, 22-23; 3:7; 4:1-3). If the believers in Ephesus follow these teachings or refuse to show love for their brothers, it will bring them spiritual deadness and perhaps also physical death. All sin brings death, but it works as a progression from spiritual death to physical death. Such sin is so serious that the believer is not commanded to pray for the sinner, though he may choose to do so.
This admonition continues John's previous discussions about the importance of loving one's brother in chapters 3-5 and his immediate admonition to pray expectantly to God (1 John 5:14-15). Grievous sins bring grievous consequences. They bring separation between the believer and God resulting in spiritual deadness, and can bring His ultimate discipline of physical death. We can pray with confidence for those committing some sins, but for others who sin more seriously, we have no assurance of answered prayer. This (Discipleship) perspective should be a warning to us about committing serious sin, and an encouragement to pray for those who are sinning.