1 John 5:16 (ESV) states, “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.”
Can we commit sin that leads to death today? And what is a sin not leading to death? And is it unloving not to pray for a situation in which sin leads to death?
Answering these questions requires us to identify the meanings of “brother” and “life” and “death.” If we follow John’s consistent use of these terms in 1 John, a brother is a fellow believer (cf. 1 John 2:9, 10, 11; 3:10, 13, 14, 15, 16, 17; 4:20, 21), life is eternal life (cf. 1 John 1:1, 2; 2:25; 3:14, 15; 5:11, 12, 13, 20), and death is eternal death (cf. 1 John 3:14; 5:17). The difficulty with keeping these definitions 1 John 5:16 is figuring out how to understand the statement that God gives life to a brother who already possesses eternal life. In other words, how does God give eternal life to someone who already has eternal life?
Some solve this dilemma by redefining the terms. The brother is a so-called professing brother but doesn’t really have eternal life and thus receives it when God gives it to him. Or, because salvation allegedly does not include one’s surrender, the sinning brother adds to his eternal life the bliss of victorious and abundant life, something apparently missing when he was initially saved or something lost because of his sin. Or, the life and death are physical, meaning that the brother sins and, if persistently unrepentant, loses his physical life at the hand of God as do those in 1 Cor 11:30, thankfully to be then brought into heaven to enjoy the eternal life he could never lose.
Realizing that good men disagree over interpreting what is admittedly a very difficult passage, I will attempt to identify “a sin not leading to death” a “sin that leads to death” by sticking as close as possible to the meaning of John’s other uses of the terms “brother,” “life,” and “death.”
A Sin Not Leading to Death
For this sin, notice that it is something observable, something “anyone sees.” At the same time, it is unspecified, leaving us to wonder what it could be. It is something ongoing, something someone is “committing” and has not committed only once. It is committed by a believer, “his brother,” that is, the brother of the one to pray for him. It is something that will end because the prayer will be answered by God’s giving of life to the sinning brother, assuming, it seems, that his repentance takes place as well.
As noted above, the rub comes when attempting to understand how eternal life is given to the brother who already possesses eternal life. The best solution I can offer to this dilemma is that, while eternal life is something experienced and present, it is also something future and promised, as said by John himself (cf. 1 John 2:25). This being said, John states that life is something God “will give” (future tense) to the sinning brother. Thus, the believer is praying that God will do what will take place, that He will give the sinning brother what is coming to him, eternal life in time to come. Assumedly, the sinning brother repents and is thereby assured that this life will indeed be his to enjoy.
Sin That Leads to Death
For this sin, it, too, is observable. By observing it, the believer knows he is not obligated to include the matter in his prayers (though he can if desired). We can assume that it is committed by an unbeliever. John does not mention a person in the statement “There is sin that leads to death.” So, if the implied sinner in this statement commits this sin leading to death, and if the death is eternal, then it cannot be committed by a believer, because believers cannot lose their eternal life (cf. John 10:28–29).
Finally, like the sin not leading to death, this sin is unspecified. While many attempt to identify this sin as high-handed sin in the OT, mortal sin, blasphemy against the Spirit, or apostasy, it nonetheless remains that John himself left it unspecified. Since 1 John is the most immediate context to search for clues, we could surmise from the passage preceding 1 John 5:16 and the letter as a whole that a sin that leads to death is the sin of rejecting Jesus as the Christ who came in the flesh to die for our sins (1 John 5:12–13; cf. 2:1–2; 4:1–6; 5:6–11). Such a sin was easy to observe because the one committing this sin eventually left the church (cf. 1 John 2:19).
So, to end on a pastoral note, when you see your brother habitually sinning in a way that is not a verbal denial of who Jesus is and it seems his faith is otherwise sincere, pray for him, and God will indeed give him all that is coming to him in his eternal life.
For the situation in which one refuses to believe Jesus is who God says He is (which obviously cannot be a brother), John does not say that you should pray for that, but it seems you are certainly welcome to do so.