When John writes that Jesus Christ “came by water and blood” (1 John 5:6a), he is using “water and blood” as a synonym for covenant. As explained in Hebrews 9:16-22, it takes blood and water, together, to bring a will/covenant into effect – the blood represents the death of something or someone. If the covenant cannot come into effect nor has any value without blood and water then, “water and blood” is synonymous with covenant. An inspired John had just found a unique way of saying, “covenant”! Thus, what John means is, this one, Jesus Christ, came by covenant—the new covenant God had made with the house of Israel and with the house of Judah, to forgive their wickedness and remember their sins no more (Heb. 8:8-12).
Not only did Jesus come by covenant, He was the sacrificial lamb of the very covenant that had brought Him to earth—to take away sin from both Jew and Gentile. John declared, when he saw Jesus, “Look, the Lamb of God, who takes away the sin of the world!" (John 1:29a: NIV). Jesus is also the mediator or High Priest who ratified the covenant (Heb. 7-8). He brought it into effect through His own body as blood and water flowed from His pierced side on Calvary’s cross (John 19:34).
God, who is faithful, has not left anything undone; He has kept His promise to forgive wickedness and sins. He gave us a Savior and High Priest, Jesus, His Son, our eternal intercessor (Heb. 7:24-25). Messiah!!