It is the Spirit who testifies, because the Spirit is the truth.
For there are three who testify: the Spirit, the water, and the blood;
and the three agree as one.
If we receive the witness of men, the witness of God is greater; for this is God’s testimony which he has testified concerning his Son.
He who believes in the Son of God has the testimony in himself. He who doesn’t believe God has made him a liar, because he has not believed in the testimony that God has given concerning his Son.
The testimony is this, that God gave to us eternal life, and this life is in his Son.
He who has the Son has the life. He who doesn’t have God’s Son doesn’t have the life.