7 “For I have not,” He says, “spoken of myself.” He says that He has not spoken of Himself, because He is not of Himself. Of this we have frequently discoursed already; so that now, without any more instruction, we have simply to remind you of it as a truth with which you are familiar. “But the Father who sent me, He gave me a commandment what I should say, and what I should speak.” We would not stay to elaborate this, did we know that we were now speaking with those with whom we have spoken on former occasions, and of these, not with all, but such only whose memories have retained what they heard: but because there are perhaps some now present who did not hear, and some in a similar condition who have forgotten what they heard, on their account let those who remember what they have heard bear with our delay.
How gives the Father a commandment to His only Son? With what words does He speak to the Word, seeing that the Son Himself is the only-begotten Word? Could it be by an angel, seeing that by Him the angels were created? Was it by means of a cloud, which, when it gave forth its sound to the Son, gave it not on His account, as He Himself also tells us elsewhere, but for the sake of others who were needing to hear it? Could it be by any sound issuing from the lips, where bodily form was wanting, and where there is no such local distance separating the Son from the Father as to admit of any intervening air, to give effect, by its percussion, to the voice, and render it audible?
Let us put away all such unworthy notions of that incorporeal and ineffable subsistence. The only Son is the Word and the Wisdom of the Father, and therein are all the commandments of the Father. For there was no time that the Son knew not the Father's commandment, so as to make it necessary for Him to possess in course of time what He possessed not before. For what He has received from the Father, He received in being born, and was given it in being begotten. For the life He is, and life He certainly received in being born, while yet there was no antecedent time when life was wanting to His personal existence.
For, on the one hand, the Father has life, and is what He has: and yet He received it not, because He is not of any one. But the Son received life as the Father's gift, of whom He is: and so He Himself is what He has; for He has life, and is the life. Listen to Himself when He says, “As the Father has life in Himself, so has He given to the Son to have life in Himself.” Could He give it to one who was in being, and yet hitherto was destitute thereof? On the contrary, in the very begetting it was given by Him who begot the life, and so life begot the life.
And to show that He begot the life equal, and not inferior to Himself, it was said, “As He has life in Himself, so has He also given to the Son to have life in Himself.” He gave life; for in begetting the life, what was it He gave Him, save to be the life? And as His nativity is itself eternal, there never was a time without that Son who is the life, and never was there a time when the Son Himself was without the life; and as His nativity is eternal, so He, who was thus born, is eternal life.
And so the Father gave not to the Son a commandment which He had not already; but, as I said, in the Wisdom of the Father, that is, in the word of the Father, are laid up all the Father's commandments. And yet the commandment is said to have been given Him, because He, to whom it is thus given, is not of Himself: and to give that to the Son which He never was without, is the same in meaning as to beget that Son who never was without existence.
Source: Tractates on the Gospel of John (New Advent)