What is the water and the blood?
The water is the testimony of God.
The blood is the testimony of Jesus the Son of God:
publicly at the Cross - where God procured our salvation, our redemption, and the remission of our sins;
privately before the Father in heaven - where Jesus secured our Atonement to the Father.
The Spirit witnessed events both on earth and in heaven, and now witnesses to both in the hearts of believers.
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And why does 1Jn 5:6 refer to then both as through [Greek dia] and in [Greek en].
Jesus came here [to Calvary] on His way to the cross through [dia] the testimony of God and the blood of Immanuel, then
Jesus came here [to the Resurrection] bringing God's testimony of our atonement to Him, that is in [en] the water and blood
. . . this being demonstrated by that very Resurrection - proof that Jesus is the Living One, and those that belong to Him live because of His Life - as He is the Way, the Truth, and the Life.
Jesus did not come bearing words only, but He demonstrated God's love by His obedience (Php 2:8).
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"The one believing in the Son of God
has the witness in himself. The one not believing God has made Him a liar, because he has not believed in the witness which God has witnessed concerning His Son" (1 John 5:10 LITV).
The believer [he who has believed into the Son] has the Spirit of God, the testimony of God, and the Blood of Jesus Christ both covering him and permeating his being; the believer comes to the Father through these, and has that very witness in himself.