"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." 1 John 5:6-8
I readily confess that I have no idea what in the world John is talking about in these verses. What does it mean that Jesus came by water and blood as opposed to just water? The list of things to which commentators suggest blood and water could be referring is long. Is it baptism and communion? A roundabout way of referring to Jesus' humanity? The blood and water that flowed from Jesus' side at his crucifixion? And what would the significance of any of those referents be? I have to honestly say I don't know ... but, of course, that won't keep me from venturing a guess.
Whenever we encounter really obscure passages of Scripture like this one, I always think its a good practice to look at the verses nearby and find something that's not obscure - something with a relatively plain and obvious meaning - and work from there. If we look just a few verses later, we find just such verses.
"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." v.11-13
There is a resounding and obvious theme in these verses: that those to whom John is writing know Jesus and therefore have eternal life. Whatever the point may be of the whole blood/water discussion, it must serve this point since this is clearly what John is driving at. The one thing John wishes to communicate to his audience here is that there should be no doubt that they have eternal life. We can ask then "What does Jesus being the one who came by blood and water as opposed to just water have to do with this truth?"
For help in answering that question, we can look to the first chapter of the Gospel of John. There, John the baptist is described as one who came by water. In v.26-27, he says "I baptize with water, but among you stands one you do not know, even he who comes after me, the strap of whose sandal I am not worthy to untie." A few verses later, John identifies this one who stands among them as Jesus, calling him "the Lamb of God, who takes away the sin of the world!" Although the rest of the Gospel of John will fill out what it means to call Jesus "Lamb of God", there can be little doubt that the mention of a lamb taking away sin would evoke thoughts of that lamb's blood being shed for just about any first century reader, especially Jewish ones. So in just a few short verses we have John the baptist presented as one who came by water contrasted with Jesus who came by water and blood. Additionally, John says in the same passage "I saw the Spirit descend from heaven like a dove, and it remained on him. I myself did not know him, but he who sent me to baptize with water said to me, 'He on whom you see the Spirit descend and remain, this is he who baptizes with the Holy Spirit.' And I have seen and have borne witness that this is the Son of God." So Jesus is described not only as having come by water and blood but also the one on whom the Spirit rests, affirming John's declaration about him.
Of course, Jesus will breathe this same Holy Spirit on his followers after his death and resurrection. This is, in fact, one of the major themes of the Gospel of John. Jesus repeatedly promises the disciples that the Holy Spirit will come to them but that this comforter can only come after Jesus has gone away; that is, after blood and water have flowed from Jesus' side at his crucifixion (John 19:34).
So what's all that got to do with John's main point that his audience has eternal life? Immediately after the verses about blood, water, and the Spirit, John's very next words are these: "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." Then follow the verses that say that this testimony God has given is that these followers of Jesus have eternal life.
In other words, we can easily imagine that there were some who were saying to these believers "You don't really have eternal life. You are just following the teaching of a mere man whose got it wrong." And John is saying in response "You know better than that! It is not the testimony of man that you have trusted but the very testimony of God that tells you that you have eternal life! Jesus is not a mere man who came by water alone as John the baptist did. Jesus is the who came to be baptized in water but who also gave his own life so that blood and water flowed from his side. And in that giving of his life the same Spirit which descended upon him in the waters of baptism now also actively bears witness among you." That is to say that the water, the blood, and the Spirit all three testify and agree to the same thing; that these believers have eternal life. The testimony of the blood and water of Jesus Christ and the testimony of the Spirit are the testimony of the one and only God and not the testimony of man. As Paul says in Romans 8:16, "The Spirit himself bear witness with our spirit that we are children of God."
"You know you have eternal life because of what the Spirit has done among you" is the message of this passage. This is typically termed the doctrine of Christian Assurance in the language of the Church; the belief that we can be assured of our salvation. But we are not assured by the witness of others about us or by our works or by the quality of our life or the "success" of our church. We are assured by the witness of God's own Spirit with our spirit. Our re-birth may be messy for sure - not unlike our first birth, a mess of blood, water, and spirit- leading one to wonder if new life is really happening in the midst of all this chaos. At times, we may even questions it ourselves as John's audience seems to have done. But in the midst of all the chaos, God reminds us by the Son and the Spirit that we are surely children of this Father.