Granting that the knowledge which they believe themselves to have acquired is somewhat greater than that of others: yet can they possess more than the belief that the Son of the Very God is Very God, seeing that in that article of the Very God every idea that is orthodox, every idea that is our salvation, is included? It includes the idea of His Goodness, His Justice, His Omnipotence: that He admits of no variableness nor alteration, but is always the same; incapable of changing to worse or changing to better, because the first is not His nature, the second He does not admit of; for what can be higher than the Highest, what can be better than the Best? In fact, He is thus associated with all perfection, and, as to every form of alteration, is unalterable; He did not on occasions display this attribute, but was always so, both before the Dispensation that made Him man, and during it, and after it; and in all His activities in our behalf He never lowered any part of that changeless and unvarying character to that which was out of keeping with it. What is essentially imperishable and changeless is always such; it does not follow the variation of a lower order of things, when it comes by dispensation to be there; just as the sun, for example, when he plunges his beam into the gloom, does not dim the brightness of that beam; but instead, the dark is changed by the beam into light; thus also the True Light, shining in our gloom, was not itself overshadowed with that shade, but enlightened it by means of itself. Well, seeing that our humanity was in darkness, as it is written, “They know not, neither will they understand, they walk on in darkness,” the Illuminator of this darkened world darted the beam of His Divinity through the whole compound of our nature, through soul, I say, and body too, and so appropriated humanity entire by means of His own light, and took it up and made it just that thing which He is Himself. And as this Divinity was not made perishable, though it inhabited a perishable body, so neither did it alter in the direction of any change, though it healed the changeful in our soul: in medicine, too, the physician of the body, when he takes hold of his patient, so far from himself contracting the disease, thereby perfects the cure of the suffering part. Let no one, either, putting a wrong interpretation on the words of the Gospel, suppose that our human nature in Christ was transformed to something more divine by any gradations and advance: for the increasing in stature and in wisdom and in favour, is recorded in Holy Writ only to prove that Christ really was present in the human compound, and so to leave no room for their surmise, who propound that a phantom, or form in human outline, and not a real Divine Manifestation, was there. It is for this reason that Holy Writ records unabashed with regard to Him all the accidents of our nature, even eating, drinking, sleeping, weariness, nurture, increase in bodily stature, growing up— everything that marks humanity, except the tendency to sin. Sin, indeed, is a miscarriage, not a quality of human nature: just as disease and deformity are not congenital to it in the first instance, but are its unnatural accretions, so activity in the direction of sin is to be thought of as a mere mutilation of the goodness innate in us; it is not found to be itself a real thing, but we see it only in the absence of that goodness. Therefore He Who transformed the elements of our nature into His divine abilities, rendered it secure from mutilation and disease, because He admitted not in Himself the deformity which sin works in the will. “He did no sin,” it says, “neither was guile found in his mouth.” And this in Him is not to be regarded in connection with any interval of time: for at once the man in Mary (where Wisdom built her house), though naturally part of our sensuous compound, along with the coming upon her of the Holy Ghost, and her overshadowing with the power of the Highest, became that which that overshadowing power in essence was: for, without controversy, it is the Less that is blest by the Greater. Seeing, then, that the power of the Godhead is an immense and immeasurable thing, while man is a weak atom, at the moment when the Holy Ghost came upon the Virgin, and the power of the Highest overshadowed her, the tabernacle formed by such an impulse was not clothed with anything of human corruption; but, just as it was first constituted, so it remained, even though it was man, Spirit nevertheless, and Grace, and Power; and the special attributes of our humanity derived lustre from this abundance of Divine Power.
There are indeed two limits of human life: the one we start from, and the one we end in: and so it was necessary that the Physician of our being should enfold us at both these extremities, and grasp not only the end, but the beginning too, in order to secure in both the raising of the sufferer. That, then, which we find to have happened on the side of the finish we conclude also as to the beginning. As at the end He caused by virtue of the Incarnation that, though the body was disunited from the soul, yet the indivisible Godhead which had been blended once for all with the subject (who possessed them) was not stripped from that body any more than it was from that soul, but while it was in Paradise along with the soul, and paved an entrance there in the person of the Thief for all humanity, it remained by means of the body in the heart of the earth, and therein destroyed him that had the power of Death (wherefore His body too is called “the Lord” on account of that inherent Godhead)— so also, at the beginning, we conclude that the power of the Highest, coalescing with our entire nature by that coming upon (the Virgin) of the Holy Ghost, both resides in our soul, so far as reason sees it possible that it should reside there, and is blended with our body, so that our salvation throughout every element may be perfect, that heavenly passionlessness which is peculiar to the Deity being nevertheless preserved both in the beginning and in the end of this life as Man. Thus the beginning was not as our beginning, nor the end as our end. Both in the one and in the other He evinced His Divine independence; the beginning had no stain of pleasure upon it, the end was not the end in dissolution.
Source: Letters (New Advent)