18 For you attribute, most godless of heretics, the birth of the Son to an act of creative will; you say that He is not born from God, but that He was created and came into existence by the choice of the Creator. And the unity of the Godhead, as you interpret it, will not allow Him to be God, for, since God remains One, the Son cannot retain His original nature in that state into which He has been born. He has been endowed, through creation, you say, with a substance different from the Divine, although, being in a sense the Only-begotten, He is superior to God's other creatures and works.
You say that He was raised up, that He in His turn might perform the task committed to Him of raising up the created world; but that His birth did not confer upon Him the Divine nature. He was born, according to you, in the sense that He came into existence out of nothing. You call Him a Son, not because He was born from God, but because He was created by God. For you call to mind that God has deemed even holy men worthy of this title, and you consider that it is assigned to the Son in exactly the same sense in which the words, I have said, You are Gods, and all of you sons of the Most High, were spoken; that is, that He bears the name through the Giver's condescension, and not by right of nature.
Thus, in your eyes, He is Son by adoption, God by gift of the title, Only-begotten by favour, First-born in date, in every sense a creature, in no sense God. For you hold that His generation was not a birth from God, in the natural sense, but the beginning of the life of a created substance.
Source: On the Trinity (New Advent)