"Perhaps some one may wish to drive us to say, that the Son is sent also by Himself, because the conception and childbirth of Mary is the working of the Trinity, by whose act of creating all things are created. And how, he will go on to ask, has the Father sent Him, if He sent Himself? To whom I answer first, by asking him to tell me, if he can, in what manner the Father has sanctified Him, if He hath sanctified Himself? For the same Lord says both; 'Do you say of him whom the Father sanctified and sent into the world, 'You are blaspheming,' because I said, 'I am the Son of God?' (Jn 10:36) while in another place He says, 'And for their sake I sanctify myself' (Jn 17:19). I ask, also, in what manner the Father delivered Him, if He delivered Himself? For the Apostle Paul says both: 'Who,' he says, 'spared not His own Son, but delivered Him up for us all' (Rm 8:32) while elsewhere he says of the Savior Himself, 'Who loved me, and delivered Himself for me' (Gal 2:20). He will reply, I suppose, if he has a right sense in these things, 'Because the will of the Father and the Son is one, and their working indivisible.' In like manner, then, let him understand the incarnation and nativity of the Virgin, through which the Son is understood as sent, to have been done by one and the same operation of the Father and of the Son indivisibly; the Holy Spirit certainly not being excluded, of whom it is expressly said, 'She was found with child by the Holy Spirit.'

"Perhaps our meaning will be more plainly unfolded, if we ask in what manner God sent His Son. The Father commanded that He should come, and the Son, complying with the command, came. Did He then request, or did He only suggest? But whichever of these it was, certainly it was done by a word, and the Word of God is the Son of God Himself. Therefore, since the Father sent Him by a word, His being sent was the work of both the Father and His Word; therefore the same Son was sent by the Father and the Son, because the Son Himself is the Word of the Father. For who would embrace so impious an opinion as to think the Father to have uttered a word in time, in order that the eternal Son might thereby be sent and might appear in the flesh in the fullness of time? But assuredly it was in that Word of God itself which was in the beginning with God and was God, namely, in the wisdom itself of God, apart from time, at what time that wisdom must appear in the flesh.

"Therefore, since without any commencement of time, the Word was in the beginning, and the Word was with God, and the Word was God, it was in the Word itself without any time, at what time the Word was to be made flesh and dwell among us (Jn 1:1-2, 14). And when this fullness of time had come, 'God sent His Son, made of a woman' (Gal 4:4), that is, made in time, that the Incarnate Word might appear to men. While it was in that Word Himself, apart from time, at what time this was to be done. For the order of times is in the eternal wisdom of God without time. Since, then, that the Son should appear in the flesh was wrought by both the Father and the Son, it is properly said that He who appeared in that flesh was sent, and that the Father who did not appear in it, sent Him; because those things which are transacted outwardly before the bodily eyes have their existence from the inward structure of the spiritual nature, and on that account are properly said to be sent."

Augustine, On the Trinity, 2.5