This is he who came by water and blood, Jesus Christ; not with the water only, but with the water and the blood. 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.
Traditional attribution
The letter does not name its author. John is the traditional attribution.