The blood and water are not only realities (res) but also, according to John, “signs” (John 20:30). Signs point to things. A stop sign points to the requirement to stop. In the Gospel of John, the signs of blood and water point to the institutions of baptism and the supper. And these two institutions themselves further evoke the realities of the cross.
When Jesus hung on the cross, Roman soldiers “pierced his side with a spear” (John 19:34). And something entirely unexpected happened. Jesus did not just bleed. Water and blood flowed from his side.
This event so impressed the early Christians that about sixty years later, John records this event and emphasized that “He who saw it has borne witness” (John 19:35). John needs his readers to know that an eyewitness saw it happen. And he emphasizes: “His testimony is true, and he knows he is telling the truth.”
Why did John (likely himself the eyewitness) so strongly remember this event sixty years in his past and want his readers to know about it for perpetuity?
John answers, “So that you also may believe.” Along with the unbroken bones of Jesus, blood and water flowing from the side of Jesus bring about belief.
And John repeats this epochal moment of salvation in history elsewhere. He writes in 1 John: “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” (1 John 5:6).
And here he makes the claim that the Spirit testifies to the blood and water of Christ. The apostolic preaching of John included the message of blood and water flowing from Christ’s side. And the Spirit testified to the veracity of his preaching.