This is the One who came by water and blood
1 John 5:6 New American Standard Bible
6 This is the One who came by water and blood, Jesus Christ; not with the water only, but with the water and with the blood. It is the Spirit who testifies, because the Spirit is the truth.
The water and blood speak of two points in which the authority of Christ was demonstrated. At Jesus baptism the Spirit of God descended upon Jesus and remained. The Father said this my Son with whom I am well pleased. It was at His water baptism that we see that Jesus is truly the anointed one.
But at His crucifixion we see that Christ is revealed as the Son of God. As He hung upon the cross, He cried out Father forgive them for they know not what they do. He cried out my God, my God why have you forsaken me. He cried out it is finished. He cried out into your hands I commit my Spirit. At the moment that Jesus gave up His Spirit and died, the curtain before the holy of holies was torn in two like a person would tear in two a mere piece of paper.
False teaching during John’s day by an individual named Cerinthus claimed that Jesus was a mere human being who only became the Christ at baptism when the Holy Spirit descended upon Him and stopped being the Christ at His crucifixion. It was an assault on the deity of Jesus Christ. Throughout the church age the devil has continually assaulted the truth of the deity of Christ. Unless a person rightly believes that Jesus is the Christ. That He is not only the Son of God, but is God, that person cannot be saved.
In the previous verse John said that those who believe that Jesus is the Son of God will overcome the world. The world John speaks of is the world system based on the lies and deceptions of Satan. The only way we overcome darkness and deception is with the light and truth of who Christ is. When our lives are built upon wrong theology, we will live lives of defeat, discouragement, and we will be imprisoned by the devil. When our lives are built upon sound doctrine, the direct result will be lives of victory, strength, freedom, and the abundant life Christ proclaimed.
It is the Holy Spirit who testifies to the truth that Jesus is the Christ. For the child of God who is in Christ, it is the inner witness of the Holy Spirit that enables us to understand who Jesus truly is. This is why we will never be able to argue anyone into God’s kingdom but are simply called to plant the truth of God’s word. As the word is planted and prayed over, the Holy Spirit will water and open the hearts and minds of people to believe that Jesus is truly the Christ, The Son of God, that He indeed is God.