6 This is the one who came by water and blood—Jesus Christ. He did not come by water only, but by water and blood. And it is the Spirit who testifies, because the Spirit is the truth. 7 For there are three that testify: 8 the Spirit, the water and the blood; and the three are in agreement.
At the beginning of Jesus’ ministry he was baptized by water. At the end he was crucified and shed his blood. Then the Holy Spirit descended, dwells within us, and testifies to Christ’s sacrifice.
Actually verses 7 is quite controversial and translated differently or even omitted from some Bibles. I checked the original Greek and the text is quite clear, and there is more than appears in verse 7 of the above NIV translation. Here's the original:
7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. 8 And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one.
Okay, then, that takes our study of this passage to a new level and also gives proof to the concept of the Trinity. I like that the Father (God), the Word (Jesus), and the Holy Ghost (Holy Spirit) bear record (of the Truth) in heaven. And these three are one.