He is the radiance of the glory of God. (Hebrews 1:3)
Since in speaking of the Son he called him eternal, and yet to those not initiated in divine things, it seemed incredible how the Son is not posterior to the one begetting him, he brings out from a kind of visible image the truth of the true doctrine about God in the words: He is the radiance of the glory. The radiance comes both from the fire and remains with the fire. It has the fire for its cause, and it is inseparable from the fire: the fire comes from the one, the radiance from the other. So if in material things it is possible for something to come from something, and to coexist with what it comes from, have no doubt (he is saying) that God the Word, the only-begotten Son of God, is both begotten as son and also coexists as Word, which is the radiance of glory, with the one begetting him. The glory comes from one, the radiance from the other. The glory remains forever, and therefore too the radiance remains forever. The radiance is of the same nature as the fire, so too the Son is as the fire.
Since in speaking of the Son he called him eternal, and yet to those not initiated in divine things, it seemed incredible how the Son is not posterior to the one begetting him, he brings out from a kind of visible image the truth of the true doctrine about God in the words: He is the radiance of the glory. The radiance comes both from the fire and remains with the fire. It has the fire for its cause, and it is inseparable from the fire: the fire comes from the one, the radiance from the other. So if in material things it is possible for something to come from something, and to coexist with what it comes from, have no doubt (he is saying) that God the Word, the only-begotten Son of God, is both begotten as son and also coexists as Word, which is the radiance of glory, with the one begetting him. The glory comes from one, the radiance from the other. The glory remains forever, and therefore too the radiance remains forever. The radiance is of the same nature as the fire, so too the Son is as the fire.
Theodoret of Cyrus, "The Letter to the Hebrews"