from the writings of St. John of Cassian against Nestorius written at the behest of Pope St. Leo (I) the Great:
For if the will of God alone, and His command sufficed to fashion the heavens, form the earth, create the sea, thrones, and seats, and angels, and archangels, and principalities, and powers, and in a word to create all the armies of heaven, and those countless thousands of thousands of the Divine hosts ("For He spoke and they were made, He commanded and they were created" ), why was it that that was insufficient for the creation of (according to you) a single man, which was sufficient for the production of all things divine, and that the power and majesty of God did not entrust that with the birth of a single infant, which had availed to fashion all things earthly and heavenly? But certainly the reason why all those works were performed by the command of God, but the nativity was only accomplished by His coming was because God could not be conceived by man unless He allowed it, nor be born unless He Himself entered in; and therefore the archangel pointed out that the sacred majesty would come upon the Virgin, I mean that as so great an event could not be brought about by human appointment, he announced that there would be present at the conception the glory of Him who was to be born. And so the Word, the Son, descended: the majesty of the Holy Ghost was present: the power of the Father was overshadowing; that in the mystery of the holy conception the whole Trinity might cooperate. "Therefore," he says, "also that holy thing which shall be born of you shall be called the Son of God." Admirably does he add "Therefore," in order to show that this would therefore follow because that had gone before; and that because God had come upon her at the conception therefore God would be present at the birth. And when the maiden understood not, he gave a reason for this great thing, saying: "Because the Holy Spirit shall come upon you, and because the power of the Most High shall overshadow you, therefore also that holy thing which shall be born shall be called the Son of God;" that is to say: That you may not be ignorant of the provision for so great a work, and the mystery of this great secret, the majesty of God shall therefore come upon you completely; because the Son of God shall be born of you. What further doubt can there be about this? Or what is there further to be said? He said that God would come upon her; that the Son of God would be born. Ask now, if you like, how the Son of God can help being God, or how she who brought forth God can fail to be Theotocos, i.e., the Mother of God? This alone ought to be enough for you; aye this ought to be amply sufficient for you.