16If anyone sees his brother commit a sin that does not lead to death, he should pray and God will give him life. I refer to those whose sin does not lead to death. There is a sin that leads to death. I am not saying that he should pray about that. 17All wrongdoing is sin, and there is sin that does not lead to death.
18We know that anyone born of God does not continue to sin; the one who was born of God keeps him safe, and the evil one cannot harm him. 19We know that we are children of God, and that the whole world is under the control of the evil one. 20We know also that the Son of God has come and has given us understanding, so that we may know him who is true. And we are in him who is true—even in his Son Jesus Christ. He is the true God and eternal life.
Commentary from Jamieson, Faussett & Brown:
17. "Every unrighteousness (even that of believers, compare 1Jo 1:9; 3:4. Every coming short of right) is sin"; (but) not every sin is the sin unto death.
and there is a sin not unto death--in the case of which, therefore, believers may intercede. Death and life stand in correlative opposition (1Jo 5:11-13). The sin unto death must be one tending "towards" (so the Greek), and so resulting in, death. ALFORD makes it to be an appreciable ACT of sin, namely, the denying Jesus to be the Christ, the Son of God (in contrast to confess this truth, 1Jo 5:1, 5), 1Jo 2:19, 22; 4:2, 3; 5:10. Such wilful deniers of Christ are not to be received into one's house, or wished "God speed." Still, I think with BENGEL, not merely the act, but also the state of apostasy accompanying the act, is included--a "state of soul in which faith, love, and hope, in short, the new life, is extinguished. The chief commandment is faith and love. Therefore, the chief sin is that by which faith and love are destroyed. In the former case is life; in the latter, death. As long as it is not evident (see on 1Jo 5:16, on 'see') that it is a sin unto death, it is lawful to pray. But when it is deliberate rejection of grace, and the man puts from him life thereby, how can others procure for him life?" Contrast Jas 5:14-18. Compare Mt 12:31, 32 as to the wilful rejection of Christ, and resistance to the Holy Ghost's plain testimony to Him as the divine Messiah. Jesus, on the cross, pleaded only for those who KNEW NOT what they were doing in crucifying Him, not for those wilfully resisting grace and knowledge. If we pray for the impenitent, it must be with humble reference of the matter to God's will, not with the intercessory request which we should offer for a brother when erring.
Pretty crazy stuff. The writer doesn't even want you to pray for this person. We are to pray for our enemies, but those who adamantly deny Christ are not to even be prayed for. This gives me some new perspective in how to pray and what to pray for. May God give me wisdom in this and to not rush to conclusions about who this truly talks about. But, by God's grace, may I not waste time (prayer or ministry) on those who already have apposed Him.