20 In many, nay almost all His discourses, He offers the explanation of this mystery, never separating Himself from the divine unity, when He confesses God the Father, and never characterising God as single and solitary, when He places Himself in unity with Him. But nowhere does He more plainly teach the mystery of His unity and His birth than when He says, But the witness which I have is greater than that of John, for the works which the Father has given Me to accomplish, the very works that I do, bear witness of Me, that the Father has sent Me, and the Father which sent Me, He has borne witness of Me.
You have neither heard His voice at any time nor seen His form. And you have not His word abiding in you, for Whom He sent, Him ye believe not. How can the Father be truly said to have borne witness of the Son, when neither He Himself was seen, nor His voice heard? Yet I remember that a voice was heard from Heaven, which said, This is My beloved Son, in Whom I have been well pleased; hear ye Him. How can it be said that they did not hear the voice of God, when the voice which they heard itself asserted that it was the Father's voice?
But perhaps the dwellers in Jerusalem had not heard what John had heard in the solitude of the desert. We must ask, then, “How did the Father bear witness in Jerusalem?” It is no longer the witness given to John, who heard the voice from heaven, but a witness greater than that of John. What that witness is He goes on to say, The works which the Father has given me to accomplish, the very works which I do, bear witness of Me, that the Father has sent Me. We must admit the authority of the testimony, for no one, except the Son sent of the Father, could do such works.
His works are therefore His testimony. But what follows? And the Father, which sent Me, He has borne witness of Me. You have neither heard His voice at any time, nor seen His form, and you have not His word abiding in you. Are they blameless, in that they did not know the testimony of the Father, Who was never heard or seen among them, and Whose word was not abiding in them? No, for they cannot plead that His testimony was hidden from them; as Christ says, the testimony of His works is the testimony of the Father concerning Him.
His works testify of Him that He was sent of the Father; but the testimony of these works is the Father's testimony; since, therefore, the working of the Son is the Father's testimony, it follows of necessity that the same nature was operative in Christ, by which the Father testifies of Him. So Christ, Who works the works, and the Father Who testifies through them, are revealed as possessing one inseparable nature through the birth, for the operation of Christ is signified to be itself the testimony of God concerning Him.
Source: On the Trinity (New Advent)