I John 5:6 “This is he that came by water and blood, even Jesus Christ; not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth.”
There are many explanations of the “water and blood”, some plausible and some are farfetched. John is explaining our relationship with our Lord in Old Testament terms. Under the Old Testament Law Service, the sacrificial shedding of “blood” illustrated the gravity of the sin for which the sacrifice was made, and it was that “blood”-Sacrifice by which our Lord saved “his people from their sins” (Matthew 1:21). Paul explains that it was the shed “blood” which, under God’s Divine Law, was required to Justify the Children of God (Romans 5:9) before the Righteous Judge (II Timothy 4:8). Water was given for refreshment (John 4:10) and for cleansing (Leviticus 1:9). Washing was constant, not only for personal hygiene, but for the perpetual cleansing of the sacrifices (Leviticus 1:13), for no filthy offering could be presented to God. All those Old Testament sacrifices pointed directly to Jesus Christ Who both cleansed us and justified us before the Righteous Judge, God the Father. The “water” and “blood” are an essential part of the “witness” to us (John 5:8) that our Lord Jesus Christ has certainly washed our sins away and has “justified” us before God the Father. Furthermore, Jesus Christ Himself is the fountain, sole source of this cleansing and refreshing and nourishing water (Numbers 20:8); and Paul explains that they “did all drink the same spiritual drink: for they drank of that spiritual Rock that followed them: and that Rock was Christ” (I Corinthians 10:4). Our Lord explained the refreshment of this Holy “water” this way, “whosoever drinketh of the water that I shall give him shall never thirst; but the water that I shall give him shall be in him a well of water springing up into everlasting life” (John 4:14). The work of the Holy Spirit is also allegorized as “water” given by Christ (John 7:37-39); this nourishing “water” (Spirit) fills us with the knowledge of God and God’s love for us and our love for Him; thus, it refreshes and nourishes us to prepare and strengthen us for His Service. Furthermore, it is the sure “witness” that we are Children of God, and that God is our Father. To make the fulfilled prophecy certain in our understanding, after our Lord “gave up the ghost” (John 19:30) “one of the soldiers with a spear pierced his side, and forthwith came there out blood and water” (John 19:34); thus, giving us the Blessed Assurance, in our souls, that our Lord Jesus Christ has completely washed our sins away and has perfectly justified us before God the Father!