John19:35
Previous VerseTranslation
And the one who saw has testified, and his testimony is true, and he knows that he is speaking truth so that you also may believe. (See comment below.)
Paraphrase
What’s more, the one who saw this happen is the same one writing this testimony about it, me, and I know that what I’m saying is true and important; I am telling you so that you too will understand what Jesus did and believe in Him.
THE STATEMENT ABOUT THE BLOOD AND WATER MUST HAVE BEEN VERY IMPORTANT
At this point John pauses to place special attention on the fact that people should believe what he just wrote because he was there, and he saw it with his own eyes. He was closer than the other disciples, and that may be one of the reasons this is not mentioned in any other Gospel. Whether or not the emphasis is intended for the entire passage about the death of Jesus, or about this detail about the blood and water flowing out can be argued either way. I think it relates primarily to the blood and water because it is a detail that no other Gospel mentioned, and the next two verses continue to talk about how prophecy was fulfilled when no bones were broken but He was pierced instead.
Why were the blood and water important?
Thank you for asking. The Law of the Former Covenant was built on the primary pillars of cleansing and atonement. Water cleanses, blood atones. These were both necessary, and both of them appear in the crucifixion narrative provided by John, the disciple who was closest to cross on which Jesus was crucified. In this Way Jesus upheld the principle that cleansing must come first, then atonement can follow.
We are cleansed when we repent and confess; our sin is atoned for when we grasp by faith the blood that Jesus shed, i.e. the life He gave.