This is the one who came through water and blood, Jesus Christ, not by water alone, but by water and blood. The Spirit is the one that testifies, and the Spirit is truth. So there are three that testify, the Spirit, the water, and the blood, and the three are of one accord.
If we accept human testimony, the testimony of God is surely greater. Now the testimony of God is this, that he has testified on behalf of his Son. Whoever believes in the Son of God has this testimony within himself. Whoever does not believe God has made him a liar by not believing the testimony God has given about his Son. And this is the testimony: God gave us eternal life, and this life is in his Son. Whoever possesses the Son has life; whoever does not possess the Son of God does not have life.