Augustine of Hippo — as excerpted in the Catena Aurea on John 1:1

“(de verb. Dom. Serm. 38. [117.] §. 6) They say, however, if He is the Son, He was born. We allow it. They rejoin: if the Son was born to the Father, the Father was, before the Son was born to Him. This the Faith rejects. Then they say, explain to us how the Son could be born from the Father, and yet be coeval with Him from whom He is born: for sons are born after their fathers, to succeed them on their death. They adduce analogies from nature; and we must endeavour likewise to do the same for our doctrine. But how can we find in nature a coeternal, when we cannot find an eternal? However, if a thing generating and a thing generated can be found any where coeval, it will be a help to forming a notion of coeternals. Now Wisdom herself is called in the Scriptures, (Wisd. 7:26) the brightness of Everlasting Light, the image of the Father. Hence then let us take our comparison, and from coevals form a notion of coeternals. Now no one doubts that brightness proceeds from fire: fire then we may consider the father of the brightness. Presently, when I light a candle, at the same instant with the fire, brightness ariseth. Give me the fire without the brightness, and I will with thee believe that the Father was without the Son. An image is produced by a mirror. The image exists as soon as the beholder appears; yet the beholder existed before he came to the mirror. Let us suppose then a twig, or a blade of grass which has grown up by the water side. Is it not born with its image? If there had always been the twig, there would always have been the image proceeding from the twig. And whatever is from another thing, is born. So then that which generates may be coexistent from eternity with that which is generated from it. But some one will say perhaps, Well, I understand now the eternal Father, the coeternal Son: yet the Son is like the emitted brightness, which is less brilliant than the fire, or the reflected image, which is less real than the twig. Not so: there is complete equality between Father and Son. I do not believe, he says; for thou hast found nothing whereto to liken it. However, perhaps we can find something in nature by which we may understand that the Son is both coeternal with the Father, and in no respect inferior also: though we cannot find any one material of comparison that will be sufficient singly, and must therefore join together two, one of which has been employed by our adversaries, the other by ourselves. For they have drawn their comparison from things which are preceded in time by the things which they spring from, man, for example, from man. Nevertheless, man is of the same substance with man. We have then in that nativity an equality of nature; an equality of time is wanting. But in the comparison which we have drawn from the brightness of fire, and the reflexion of a twig, an equality of nature thou dost not find, of time thou dost. In the Godhead then there is found as a whole, what here exists in single and separate parts; and that which is in the creation, existing in a manner suitable to the Creator.”