Jesus Christ--he is the one who came by water and blood, not by water only, but by water and by blood. And the Spirit is the one who testifies, because the Spirit is the truth.
For there are three that testify:
the Spirit, the water, and the blood--and these three are in agreement.
If we accept human testimony, God's testimony is greater, because it is God's testimony that he has given about his Son.
The one who believes in the Son of God has this testimony within himself. The one who does not believe God has made him a liar, because he has not believed in the testimony God has given about his Son.
And this is the testimony: God has given us eternal life, and this life is in his Son.
The one who has the Son has life. The one who does not have the Son of God does not have life.
I have written these things to you who believe in the name of the Son of God so that you may know that you have eternal life.