“Thus the soldiers went and with respect to the first, they broke the legs and also of the other who was crucified with him. But when they came to Jesus, they saw that he was already a dead man, and so they did not break his legs. Instead, one of the soldiers took his spear and pierced his side. What flowed out immediately was blood and water. The one who saw it bears witness and the witness is the Truth. He knows what he says to be the truth and it is in order that you might believe. For this happened in order that the Scripture might be fulfilled: ‘none of his bones were broken.’ And again, likewise the Scripture says, ‘They looked on the one they pierced.’”
As we have continually seen with respect to this account of our Lord’s crucifixion, over and over again there is an emphasis on the fulfillment of the Scriptures. These Old Testament prophecies were put in place by God for the express purpose of demonstrating his sovereignty over history and who his Messiah would be. In this case, there is a reference to Exodus 12:46 and Psalm 34:20 with respect to Jesus’ bones not being broken and then to Zechariah 12:10 as to them looking on him whom they have pierced.
A great deal of debate has circulated around the question of the blood and water that flowed from Jesus’ side. I have read accounts by doctors who point out that the pericardium, were it pierced at the point of death or shortly thereafter, would issue water as well as blood. And this may very well be true, though as anatomy is not my field of expertise, I will leave that to those more qualified to debate.
What I can speak to is the text and John’s statement in the verse that follows is very important. He repeatedly affirms that he witnessed this event and that his testimony is true so that we might believe. Believe what? If the purpose of John’s book is to be of any help to us (found in John 20:30-31), then the answer is clear; he writes so that we may believe that Jesus is the Christ and by believing, we “might have life.” In other words, this water and blood, in John’s eyes, is a testimony to the saving work of Jesus Christ, the Son of God, not simply that he was dead.
So, what then does the water and blood mean? Some have pointed to Communion and the water of baptism and the blood represented in the cup, though I think that is a bit of a stretch to infer. Others connect the water and the blood with the purification ritual mentioned in Hebrews 9:19, which is an allusion to Leviticus 14:1-9. This passage in Leviticus is a reference to the ceremonial cleansing of one with leprosy, not the atonement for sin as the author of Hebrews is applying the text. That said, the context of Hebrews 9 is such where leprosy can be seen as a fit analogy for the defilement of our sin before God, making this answer, likely the best.
It is indeed true that sprinkled blood was used in other contexts when it came to purify from sin as was the sprinkling with water, but the language of cleansing from leprosy seems to be where they are brought together. If one is to appeal to the imagery of leprosy, it brings to mind the old hymn that goes:
“Lord, now indeed I find,
Thy power and Thine alone,
Can change the leper’s spots,
And melt the heart of stone…”
— Elvina Hall