16 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. 17 All wrongdoing is sin, but there is sin that does not lead to death. 1 John 5:16-17
John has taught his readers to imitate Jesus by obeying the Father, as He has obeyed (2:6, 3:6), and by showing love to our brothers as Christ has shown love to us (3:16). Here he calls on them to intercede with God for those who are sinning. This is an indirect call to imitate Jesus as He is an intercessor, He pleads with God on our behalf. As the Apostle Paul says, “Christ Jesus is the one who died—more than that, who was raised—who is at the right hand of God, who indeed is interceding for us (Romans 8:34).” It is not our intercession that gives life. It is God that gives life (v. 16), and His righteous anger is only turned aside by Christ’s propitiation (2:2). But John is talking about brothers, those who are already followers of Christ. So, in a sense, their sin is already forgiven in Christ, and God is pleased to hear our requests because of Him.
John warns that there is an exception, though, in the “sin that leads to death.” This must be spiritual death, of the type Jesus talks about in the Gospel of Mark:
“Truly, I say to you, all sins will be forgiven the children of man, and whatever blasphemies they utter, but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin (Mark 3:28-29).”