For there are three that testify:
the Spirit, the water, and the blood-- and these three are in agreement.
If we accept the testimony of men, 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 the testimony in himself. The one who does not believe God has made Him a liar, because he has not believed in the testimony that God has given about His Son.)
And this is the testimony: God has given us eternal life, and this life is in His Son.