He is the radiance of the glory of God and the exact imprint of his nature, and he upholds the universe by the word of his power. (Hebrews 1:3)
And for this reason also Paul calls Him “the brightness of glory,” that we may learn that as the light from the lamp is of the nature of that which sheds the brightness, and is united with it (for as soon as the lamp appears the light that comes from it shines out simultaneously), so in this place the apostle would have us consider both that the Son is of the Father, and that the Father is never without the Son, for it is impossible that glory should be without radiance, as it is impossible that the lamp should be without brightness. But it is clear that as His being brightness is a testimony to His being in relation with the glory (for if the glory did not exist, the brightness shed from it would not exist), so, to say that the brightness “once was not” is a declaration that the glory also was not, when the brightness was not, for it is impossible that the glory should be without the brightness.
As therefore it is not possible to say in the case of the brightness, “If it was, it did not come into being, and if it came into being it was not,” so it is in vain to say this of the Son, seeing that the Son is the brightness. Let those also who speak of “less” and “greater,” in the case of the Father and the Son, learn from Paul not to measure things immeasurable. For the apostle says that the Son is the express image of the Person of the Father. It is clear then that however great the Person of the Father is, so great also is the express image of that Person, for it is not possible that the express image should be less than the Person contemplated in it.
And this the great John also teaches when he says, “In the beginning was the Word, and the Word was with God.” For in saying that he was “in the beginning” and not “after the beginning,” he showed that the beginning was never without the Word; and in declaring that “the Word was with God,” he signified the absence of defect in the Son in relation to the Father, for the Word is contemplated as a whole together with the whole being of God. For if the Word were deficient in His own greatness so as not to be capable of relation with the whole being of God, we are compelled to suppose that that part of God which extends beyond the Word is without the Word. But in fact the whole magnitude of the Word is contemplated together with the whole magnitude of God: and consequently in statements concerning the Divine nature, it is not admissible to speak of “greater” and “less.”
And for this reason also Paul calls Him “the brightness of glory,” that we may learn that as the light from the lamp is of the nature of that which sheds the brightness, and is united with it (for as soon as the lamp appears the light that comes from it shines out simultaneously), so in this place the apostle would have us consider both that the Son is of the Father, and that the Father is never without the Son, for it is impossible that glory should be without radiance, as it is impossible that the lamp should be without brightness. But it is clear that as His being brightness is a testimony to His being in relation with the glory (for if the glory did not exist, the brightness shed from it would not exist), so, to say that the brightness “once was not” is a declaration that the glory also was not, when the brightness was not, for it is impossible that the glory should be without the brightness.
As therefore it is not possible to say in the case of the brightness, “If it was, it did not come into being, and if it came into being it was not,” so it is in vain to say this of the Son, seeing that the Son is the brightness. Let those also who speak of “less” and “greater,” in the case of the Father and the Son, learn from Paul not to measure things immeasurable. For the apostle says that the Son is the express image of the Person of the Father. It is clear then that however great the Person of the Father is, so great also is the express image of that Person, for it is not possible that the express image should be less than the Person contemplated in it.
And this the great John also teaches when he says, “In the beginning was the Word, and the Word was with God.” For in saying that he was “in the beginning” and not “after the beginning,” he showed that the beginning was never without the Word; and in declaring that “the Word was with God,” he signified the absence of defect in the Son in relation to the Father, for the Word is contemplated as a whole together with the whole being of God. For if the Word were deficient in His own greatness so as not to be capable of relation with the whole being of God, we are compelled to suppose that that part of God which extends beyond the Word is without the Word. But in fact the whole magnitude of the Word is contemplated together with the whole magnitude of God: and consequently in statements concerning the Divine nature, it is not admissible to speak of “greater” and “less.”
Gregory of Nyssa, On the Faith