I write these things to you who believe in the name of the Son of God, that you may know that you have eternal life. And this is the confidence that we have toward him, that if we ask anything according to his will he hears us. And if we know that he hears us in whatever we ask, we know that we have the requests that we have asked of him. 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. All wrongdoing is sin, but there is sin that does not lead to death. We know that everyone who has been born of God does not keep on sinning, but he who was born of God protects him, and the evil one does not touch him. We know that we are from God, and the whole world lies in the power of the evil one. And we know 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, in his Son Jesus Christ. He is the true God and eternal life. Little children, keep yourselves from idols. (1 John 5:13–21)
And so to the end of 1 John, and the apostle gives us the reason why he put pen to paper. The believers in the church had been shaken by the presence of some false teachers. They were doubting that they had eternal life. They had lost confidence.
John wants his readers to have an assurance of their place with God, and not to be shaken. If you “believe in the name of the Son of God”, then you can be confident in your relation to God. Your feelings or experiences should not shake that. He hears us when we pray, and protects us. We may still sin - something which causes us remorse - but our position with God relies upon Jesus’s work, and not ours.
As long as we are not guilty of the “sin which leads to death”, then we can be assured of our place before God. Which of course begs the question: what is the sin which leads to death?
I have read a lot of commentators on this passage, and have read a lot of opinions. It seems to me that the best way to understand this is not to focus on the ‘sin’ (is it x, y or z) but to focus on the one committing the sin. Having now read through this letter we have seen that those who “believe in the name of the Son of God” can be confident of eternal life, so the sin leading to death would appear to be sins committed by those who do not believe in Christ. In those cases it is better to pray that the one sinning would come to believe in Christ, than to pray they do not continue to sin. For John, everything flows from what you believe in. And the one to believe in is “him who is true, in his Son Jesus Christ”.
“He is the true God and eternal life”. Amen!