5 What shall we say then, brethren? How spoke the Father to the Son, seeing that the Son says, “As the Father taught me, I speak these things”? Did He speak to Him? When the Father taught the Son, did He use words, as you do when you teach your son? How could He use words to the Word! What words, many in number, could be used to the one Word? Did the Word of the Father approach His ears to the Father's mouth? Such things are carnal: banish them from your hearts. For this I say, if only you have understood my words, I certainly have spoken and my words have sounded, and by their sound have reached your ears, and through your sense of hearing have carried their meaning to your mind, if so be you have understood.
Suppose that some person of Latin speech has heard, but has only heard without understanding, what I have said. As regards the noise issuing from my mouth, he who has understood not has been a sharer therein just like yourselves. He has heard that sound; the same syllables have smote on his ears, but they have produced no effect on his mind. Why? Because he understood not. But if you have understood, whence comes your understanding? My words have sounded in the ear: have I kindled any light in the heart?
Without doubt, if what I have said is true, and this truth you have not only heard, but also understood, two things have there been wrought (distinguish between them), hearing and intelligence. Hearing has been wrought by me, but by whom has understanding? I have spoken to the ear, that you might hear; who has spoken to your heart for understanding? Doubtless some one has also said something to your heart, that not only the noise of words might strike your ear, but something also of the truth might descend into your heart.
Some one has spoken also to your heart, but you do not see him. If, brethren, you have understood, your heart also has been spoken to. Intelligence is the gift of God. And who, if you have understood, has spoken so in your heart, but He to whom the Psalm says, “Give me understanding, that I may learn Your commandments?” For example, the bishop has spoken. What has he said, some one asks. You repeat what he has spoken, and add, He has said the truth. Then another, who has not understood, says, What has he said, or what is it you are praising?
Both have heard me; I have spoken to both; but to one of them God has spoken. If we may compare small things with great (for what are we to Him?), something, I know not what, of an incorporeal and spiritual kind God works in us, which is neither sound to strike the ear, nor color to be discerned by the eyes, nor smell to enter the nostrils, nor taste to be judged of by the mouth, nor anything hard or soft to be sensible to the touch; yet something there is which it is easy to feel—impossible to explain.
If then God, as I was saying, speaks in our hearts without sound, how speaks He to His Son? Thus then, brethren, think thus as much as you can, if, as I have said, we may in some measure compare small things with great: think thus. In an incorporeal way the Father spoke to the Son, because in an incorporeal way the Father begot the Son. Nor did He so teach Him as if He had begotten Him untaught; but to have taught Him is the same as to have begotten Him full of knowledge; and this, “The Father has taught me,” is the same as, The Father has begotten me already knowing.
For if, as few understand, the nature of the Truth is simple, to be is to the Son the same as to know. From Him therefore He has knowledge, from whom He has being. Not that from Him He had first being, and afterwards knowledge; but as in begetting He gave Him to be, so in begetting He gave Him to know; for, as was said, to the simple nature of the Truth, being is not one thing and knowing another, but one and the same.
Source: Tractates on the Gospel of John (New Advent)