What John seems to be doing in 1John 5:6-8 is to establish a legal witness for what he has claimed about the Lord, Jesus. He, Jesus, came by water and blood, which is to say, not by water alone but by water and blood (1John 5:6), meaning two witnesses. He didn’t merely come to teach us a doctrine to follow, but in teaching us he also lived that doctrine out (John 13:34), showing us who God is, and how we should behave, having that knowledge. In other words, he showed us how to love God (obedience) and how we should love our brethren. In giving us an example, Jesus claimed it fell to us to do as he did (John 13:15), if we claimed to love him (John 8:42; 14:15).
John also mentioned a third witness, the Spirit, which he concluded is truth (1John 5:6, 8). Jesus referred to this as the Spirit of truth, meaning truth was the characteristic of the Spirit that comes from the Father (John 15:26), and no lie comes from truth (1John 2:21). Therefore, the Spirit, which comes from the Father, abides in the believer and teaches him the truth about the Lord and how to live in our understanding of him (1John 2:27), and the three witnesses agree. That is, there is no contradiction between what Jesus said and did and what the Spirit testifies to the believer (1John 5:8).
John also told his readers that the three witnesses, which constitute the legal witness of the Lord (Deuteronomy 17:6; 19:15; Matthew 18:16 and 2Corinthians 13:1), can be understood as the testimony of man and the testimony of God (1John 5:9), which seems to be saying that the water and the blood constitute the witness of man, while the testimony of the Spirit constitutes the witness of God. At first, one might conclude that, if such a thing were true, Jesus couldn’t, himself, be God, because he embodied the witness of the water and blood, which is a man’s witness, according to this argument (1John 5:6-9). Yet, our understanding of John’s statement couldn’t mean that, because John also testified in his Gospel narrative that the one who became Jesus is God (John 1:1, 14). Moreover, Paul also testifies the same thing:
And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory. (1 Timothy 3:16)
Paul tells us that, according to the mystery of godliness, GOD was manifest in the flesh (John 1:1, 14) and was seen by the messengers, meaning the Apostles and writers of the New Covenant text (angels in the text, but it refers to men not spirit beings), believed on in the world (i.e. the Gospel was preached and believed), and received up into glory, or, in other words the God who was manifest ascended into heaven (Acts 1:9). So, if it is true that God became man (1John 1:1, 14), whatever his witness might have been, it was the witness of God, no matter what form he took to give such a witness. How, then, should we understand John’s inference that the witness of water and blood is the testimony of men, but the witness of the Spirit is the testimony of God?
Actually, what John is referring to is the record he preached (and the other Apostles and evangelists who knew Jesus, while he was on earth). Folks believe his witness about the water and blood, or about what Jesus said and did. This was the Gospel, and their testimony was worthy of acceptance, because it represented a valid witness of what many had seen and heard (cp. 1John 1:1, 3). Their testimony was the valid witness of the water and blood, which when properly understood represents what Jesus, who was God manifest (John 1:1, 14), said and did. Therefore, the witness of man (1John 5:9) is the Gospel about Jesus, namely, what his appointed disciples said about him. Nevertheless, greater than this is the witness of the Spirit (1John 5:9; cp. 1John 2:27), which is Christ in the believer, our hope of glory (Colossians 1:27).
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