8 There follow the words: “And I know that His commandment is life everlasting.” If, then, the Son Himself is eternal life, and the Father's commandment the same, what else is expressed than this, I am the Father's commandment? And in like manner, in what He proceeds to say, “Whatsoever I speak, even as the Father said unto me, so I speak,” let us not be taking the “said unto me” as if the Father used words in speaking to the only Word, or that the Word of God needed words from God.
The Father spoke to the Son in the same way as He gave life to the Son; not that He knew not the one, or had not the other, but just because He was the Son. What, then, do the words mean, “Even as He said unto me, so I speak;” but just, I speak the truth? So the former said as the Truthful One what the latter thus spoke as the Truth. The Truthful begot the Truth. What, then, could He now say to the Truth? For the Truth had no imperfection to be supplied by additional truth.
He spoke, therefore, to the Truth, because He begot the Truth. And in like manner the Truth Himself speaks what has been said to Him; but only to those who have understanding, and who are taught by Him as the God-begotten Truth. But that men might believe what they had not yet capacity to understand, words that were audible issued from His human lips; sounds passing rapidly away broke on the ear, and speedily completed the little term of their duration: but the truths themselves, of which the sounds are but signs, passed, as it were, into the memory of those who heard them, and have come down to us also by means of written characters as signs addressed to the eye.
But it is not thus that the Truth speaks; He speaks inwardly to the souls of the intelligent; He needs no sound to instruct, but floods the mind with the light of understanding. And he, then, who in that light is able to behold the eternity of His birth, himself hears in the same way the Truth speaking, as He heard the Father telling Him what He should speak. He has awakened in us a great longing for that sweet experience of His presence within; but it is by daily growth that we acquire it; it is by walking that we grow, and it is by forward efforts we walk, so as to be able at last to attain it.
Source: Tractates on the Gospel of John (New Advent)