Hebrews 1:3
But the very thing which he said, “the brightness of the glory,” hear also Christ Himself saying, “I am the Light of the world.” Therefore he [the Apostle] uses the word “brightness,” showing that this was said in the sense of “Light of Light.” Nor is it this alone which he shows, but also that He has enlightened our souls; and He has Himself manifested the Father, and by “the brightness” he has indicated the nearness of the Being [of the Father and the Son]. Observe the subtlety of his expressions. He has taken one essence and subsistence to indicate two subsistences. Which he also does in regard to the knowledge of the Spirit; for as he says that the knowledge of the Father is one with that of the Spirit, as being indeed one, and in nought varying from itself: so also here he has taken hold of one certain [thing] whereby to express the subsistence of the Two.
And he adds that He is “the express Image.” For the “express Image” is something other than its Prototype: yet not Another in all respects, but as to having real subsistence. Since here also the term, “express image,” indicates there is no variation from that whereof it is the “express image”: its similarity in all respects. When therefore he calls Him both Form, and express Image, what can they say? “Yea,” says he, “man is also called an Image of God.” What then! Is he so [an image of Him] as the Son is? No (says he) but because the term, image, does not show resemblance. And yet, in that man is called an Image, it shows resemblance, as in man. For what God is in Heaven, that man is on earth, I mean as to dominion. And as he has power over all things on earth, so also has God power over all things which are in heaven and which are on earth. But otherwise, man is not called “Express image,” he is not called Form: which phrase declares the substance, or rather both substance and similarity in substance. Therefore just as “the form of a slave” expresses no other thing than a man without variation [from human nature], so also “the form of God” expresses no other thing than God.
“Who being” (says he) “the brightness of His glory.” See what Paul is doing. Having said, “Who being the brightness of His glory,” he added again, “He sat down on the right hand of the Majesty”: what names he has used, nowhere finding a name for the Substance. For neither “the Majesty,” nor “the Glory” sets forth the Name, which he wishes to say, but is not able to find a name. For this is what I said at the beginning, that oftentimes we think something, and are not able to express [it]: since not even the word God is a name of substance, nor is it at all possible to find a name of that Substance.
And what marvel, if it be so in respect of God, since not even in respect of an Angel, could one find a name expressive of his substance? Perhaps too, neither in respect of the soul. For this name [soul] does not seem to me to be significative of the substance thereof, but of breathing. For one may see that the same [thing] is called both Soul and Heart and Mind: for, says he, “Create in me a clean heart, O God”, and one may often see that it [the soul] is called spirit.
“And upholding all things by the word of His power.” Tell me, “God said” (it is written), “Let there be light”: “the Father, says one, commanded, and the Son obeyed”? But behold here He also [the Son] acts by word. For (says he), “And upholding all things”— that is, governing; He holds together what would fall to pieces; For, to hold the world together, is no less than to make it, but even greater (if one must say a strange thing). For the one is to bring forward something out of things which are not: but the other, when things which have been made are about to fill back into non-existence, to hold and fasten them together, utterly at variance as they are with each other: this is indeed great and wonderful, and a certain proof of exceeding power.
Then showing the easiness, he said, “upholding”: (he did not say, governing, from the figure of those who simply with their finger move anything, and cause it to go round.) Here he shows both the mass of the creation to be great, and that this greatness is nothing to Him. Then again he shows the freedom from the labor, saying, “By the word of His power.” Well said he, “By the word.” For since, with us, a word is accounted to be a bare thing, he shows that it is not bare with God. But, how “He upholds by the word,” he has not further added: for neither is it possible to know. Then he added concerning His majesty: for thus John also did: having said that “He is God”, he brought in the handiwork of the Creation. For the same thing which the one indirectly expressed, saying, “In the beginning was the Word,” and “All things were made by Him”, this did the other also openly declare by “the Word,” and by saying “by whom also He made the worlds.” For thus he shows Him to be both a Creator, and before all ages. What then? When the prophet says, concerning the Father, “You are from everlasting and to everlasting”, and concerning the Son, that He is before all ages, and the maker of all things— what can they say? Nay rather, when the very thing which was spoken of the Father—“He which was before the worlds,”— this one may see spoken of the Son also? And that which one says, “He was life”, pointing out the preservation of the creation, that Himself is the Life of all things—so also says this other, “and upholding all things by the word of His power”: not as the Greeks who defraud Him, as much as in them lies, both of Creation itself, and of Providence, shutting up His power, to reach only as far as to the Moon.
“By Himself” (says he) “having purged our sins.” Having spoken concerning those marvelous and great matters, which are most above us, he proceeds to speak also afterwards concerning His care for men. For indeed the former expression, “and upholding all things,” also was universal: nevertheless this is far greater, for it also is universal: for, for His part, “all” men believed. As John also, having said, “He was life,” and so pointed out His providence, says again, and “He was light.”
“By Himself,” says he, “having purged our sins, He sat down on the right hand of the Majesty on high.” He here sets down two very great proofs of His care: first the “purifying us from our sins,” then the doing it “by Himself.” And in many places, you see him making very much of this—not only of our reconciliation with God, but also of this being accomplished through the Son. For the gift being truly great, was made even greater by the fact that it was through the Son.
For in saying, “He sat on the right hand,” and, “having by Himself purged our sins,”— though he had put us in mind of the Cross, he quickly added the mention of the resurrection and ascension. And see his unspeakable wisdom: he said not, “He was commanded to sit down,” but “He sat down.” Then again, lest you should think that He stands, he subjoins, “For to which of the angels said He at any time, Sit on My right hand.”
Source: Homilies on the Epistle to the Hebrews (New Advent)