This is the one who came by water and blood: Jesus Christ. Not by water only but by water and blood. And the Spirit is the one who testifies, because the Spirit is the truth.
The three are testifying—
the Spirit, the water, and the blood—and the three are united in agreement.
If we receive human testimony, God's testimony is greater, because this is what God testified: he has testified about his Son.
The one who believes in God's Son has the testimony within; the one who doesn't believe God has made God a liar, because that one has not believed the testimony that God gave about his Son.
And this is the testimony: God gave eternal life to us, and this life is in his Son.
The one who has the Son has life. The one who doesn't have God's Son does not have life.