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“This is he who came by water and blood—Jesus Christ; not by the water only but by the water and the blood. And the Spirit is the one who testifies, because the Spirit is the truth. For there are three that testify: the Spirit and the water and the blood; and these three agree. If we receive the testimony of men, the testimony of God is greater, for this is the testimony of God that he has borne concerning his Son. Whoever believes in the Son of God has the testimony in himself. Whoever does not believe God has made him a liar, because he has not believed in the testimony that God has borne concerning his Son. And this is the testimony, that God gave us eternal life, and this life is in his Son. Whoever has the Son has life; whoever does not have the Son of God does not have life. 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.” (1 John 5:6–17, ESV) 

The text seems to indicate that there was a disagreement about Jesus. John reminds his readers that Jesus came by water which may point back to his baptism where he was commissioned and empowered for his work. Jesus also came by blood which points to his death and the place where he declared that his work was finished. The Holy Spirit points to the Son, and the Son points to the Father. This testimony is greater for if human testimony is established by only two or three witnesses, then how much greater is the testimony of God? If one accepts the testimony of God, then they have the Son and if they have the Son, then they have eternal life. 

John has written these things to believers that have thus far overcome the secessionists so that they might be filled with confidence and know that they have eternal life. The arrangement is to remember what they heard from the beginning about Jesus, believe what they heard, live because they believe and know because the evidence and the internal testimony give the assurance of eternal life. Our union with Christ also assures the believers that God hears our prayers. What a marvelous thought. God listens to his children in spite of their failure, rebellion, and mistakes. He hears, and he answers. Pray is not for our benefit and God is not coerced. You will not tell God anything he does not already know, but it is by prayer that we seek God's will and embrace it. 

Verses sixteen and seventeen can be a bit confusing. What is the sin that does not lead to death and what is the sin that leads to death? Is John saying that someone can lose eternal life? Certainly not. I think they key to understanding this is found in the term brother. The term brother may be looked upon as one who claims to have fellowship and be a brother but in fact, they are dead in their trespasses and sins. Both of these are dead, but those who have rejected Jesus will die eternally. John Stott offers some helpful insight: 

In John’s view, they were not apostates; they were counterfeits. They were not true ‘brothers’ who had received eternal life and subsequently forfeited it. They were ‘antichrists’. Denying the Son, they did not possess the Father (2:22–23; 2 John 9). They were children of the devil, not children of God (3:10). True, they had once been members of the visible congregation and had then no doubt passed as ‘brothers’. But they went out, and by their withdrawal it was made evident that they ‘did not really belong to us’ (2:19). Since they rejected the Son, they forfeited life (5:12). Their sin did indeed lead to death.
— Stott, John R. W. The Letters of John: An Introduction and Commentary. Vol. 19. Downers Grove, IL: InterVarsity Press, 1988. Print. Tyndale New Testament Commentaries.

All sin is a violation of God's law and justice and therefore deserves death. Some sin will not result in death when a sinner repents and trusts the finished work of Christ. If you have the Son, you have eternal life. John offers assurance that though they may fall into sin from time to time, their sins do not lead to death because of the work and ministry of Jesus.