7 My opinion is, however, that in this passage the Apostle's argument is directed to a different end; and it is looking to this that he uses the terms “brightness of glory,” and “express image of person.” Whoever keeps this carefully in view will find nothing that clashes with what I have said, but that the argument is conducted in a special and peculiar sense. For the object of the apostolic argument is not the distinction of the hypostases from one another by means of the apparent notes; it is rather the apprehension of the natural, inseparable, and close relationship of the Son to the Father.
He does not say “Who being the glory of the Father” (although in truth He is); he omits this as admitted, and then in the endeavour to teach that we must not think of one form of glory in the case of the Father and of another in that of the Son, He defines the glory of the Only-begotten as the brightness of the glory of the Father, and, by the use of the example of the light, causes the Son to be thought of in indissoluble association with the Father. For just as the brightness is emitted by the flame, and the brightness is not after the flame, but at one and the same moment the flame shines and the light beams brightly, so does the Apostle mean the Son to be thought of as deriving existence from the Father, and yet the Only-begotten not to be divided from the existence of the Father by any intervening extension in space, but the caused to be always conceived of together with the cause.
Precisely in the same manner, as though by way of interpretation of the meaning of the preceding cause, and with the object of guiding us to the conception of the invisible by means of material examples, he speaks also of “express image of person.” For as the body is wholly in form, and yet the definition of the body and the definition of the form are distinct, and no one wishing to give the definition of the one would be found in agreement with that of the other; and yet, even if in theory you separate the form from the body, nature does not admit of the distinction, and both are inseparably apprehended; just so the Apostle thinks that even if the doctrine of the faith represents the difference of the hypostases as unconfounded and distinct, he is bound by his language to set forth also the continuous and as it were concrete relation of the Only-begotten to the Father.
And this he states, not as though the Only-begotten had not also a hypostatic being, but in that the union does not admit of anything intervening between the Son and the Father, with the result that he, who with his soul's eyes fixes his gaze earnestly on the express image of the Only-begotten, is made perceptive also of the hypostasis of the Father. Yet the proper quality contemplated in them is not subject to change, nor yet to commixture, in such wise as that we should attribute either an origin of generation to the Father or an origin without generation to the Son, but so that if we could compass the impossibility of detaching one from the other, that one might be apprehended severally and alone, for, since the mere name implies the Father, it is not possible that any one should even name the Son without apprehending the Father.
Source: Letters (New Advent)