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. For there are three that testify: the Spirit and the water and the blood; and the three are in agreement.
(Verse 7 is disputed as it is not in early Greek manuscripts and may have begun as a note in the margin which was later copied into the text. If it is genuine it likely refers to the Trinity.)
Probably with those teaching falsely that Christ was a spiritual being and not flesh, John gives evidence that Jesus is the Son of God. John himself witnessed the water baptism of Jesus and His shedding of blood on the cross. Theologians disagree over the specific meaning of the verse. Does water mean purity, human birth, baptism, or the water that flowed from Jesus’ side when pierced to prove His death? Most would agree that “blood” alludes to His death, so I believe “water” refers to the baptism that marked the beginning of his ministry and signified His purity, although He needed no cleansing. Water had been symbolic of cleansing in the Old Testament and Jewish religion – ablutions were common in ceremonial law. Blood was clearly familiar to religion of the day as while the Temple stood the Jews daily remitted blood sacrifices for their sins, and Greek religion ordained animal sacrifice as well. Jesus came to fulfill the law through the water and the blood.
The Spirit bore witness in miraculous ways through the spiritual gifts and miraculous powers given to the apostles and early believers, but He still bears witness today. He testifies in every great religious revival and in the moment of initial faith for each believer. “No one can say, "Jesus is Lord," except by the Holy Spirit” (1 Cor. 12:3). The Spirit testifies as He changes our carnal disposition into a new life reflecting Christ in us and has always revealed God through our understanding of Holy Scripture.
Many see the ordinances of baptism and the Lord’s Supper alluded to in the last verse, and certainly these acts of worship are a visual representation of the work of Jesus in our lives.