It is, then, completely in keeping with this, that He Who was thus pouring Himself into our nature should accept this commixture in all its accidents. For as they who wash clothes do not pass over some of the dirt and cleanse the rest, but clear the whole cloth from all its stains, from one end to the other, that the cloak by being uniformly brightened from washing may be throughout equal to its own standard of cleanness, in like manner, since the life of man was defiled by sin, in its beginning, end, and all its intermediate states, there needed an abstergent force to penetrate the whole, and not to mend some one part by cleansing, while it left another unattended to.
For this reason it is that, seeing that our life has been included between boundaries on either side, one, I mean, at its beginning, and the other at its ending, at each boundary the force that is capable of correcting our nature is to be found, attaching itself to the beginning, and extending to the end, and touching all between those two points. Since, then, there is for all men only one way of entrance into this life of ours, from whence was He Who was making His entrance among us to transport Himself into our life?
From heaven, perhaps some one will say, who rejects with contempt, as base and degraded, this species of birth, i.e. the human. But there was no humanity in heaven: and in that supramundane existence no disease of evil had been naturalized; but He Who poured Himself into man adopted this commixture with a view to the benefit of it. Where, then, evil was not and the human life was not lived, how is it that any one seeks there the scene of this wrapping up of God in man, or, rather, not man, but some phantom resemblance of man?
In what could the recovery of our nature have consisted if, while this earthly creature was diseased and needed this recovery, something else, among the heavenly beings, had experienced the Divine sojourning? It is impossible for the sick man to be healed, unless his suffering member receives the healing. If, therefore, while this sick part was on earth, omnipotence had touched it not, but had regarded only its own dignity, this its pre-occupation with matters with which we had nothing in common would have been of no benefit to man.
And with regard to the undignified in the case of Deity we can make no distinction; that is, if it is allowable to conceive at all of anything beneath the dignity of Deity beside evil. On the contrary, for one who forms such a narrow-minded view of the greatness of the Deity as to make it consist in inability to admit of fellowship with the peculiarities of our nature, the degradation is in no point lessened by the Deity being conformed to the fashion of a heavenly rather than of an earthly body.
For every created being is distant, by an equal degree of inferiority, from that which is the Highest, Who is unapproachable by reason of the sublimity of His Being: the whole universe is in value the same distance beneath Him. For that which is absolutely inaccessible does not allow access to some one thing while it is unapproachable by another, but it transcends all existences by an equal sublimity. Neither, therefore, is the earth further removed from this dignity, nor the heavens closer to it, nor do the things which have their existence within each of these elemental worlds differ at all from each other in this respect, that some are allowed to be in contact with the inaccessible Being, while others are forbidden the approach.
Otherwise we must suppose that the power which governs the Universe does not equally pervade the whole, but in some parts is in excess, in others is deficient. Consequently, by this difference of less or more in quantity or quality, the Deity will appear in the light of something composite and out of agreement with itself; if, that is, we could suppose it, as viewed in its essence, to be far away from us, while it is a close neighbour to some other creature, and from that proximity easily apprehended.
But on this subject of that exalted dignity true reason looks neither downward nor upward in the way of comparison; for all things sink to a level beneath the power which presides over the Universe: so that if it shall be thought by them that any earthly nature is unworthy of this intimate connection with the Deity, neither can any other be found which has such worthiness. But if all things equally fall short of this dignity, one thing there is that is not beneath the dignity of God, and that is, to do good to him that needed it. If we confess, then, that where the disease was, there the healing power attended, what is there in this belief which is foreign to the proper conception of the Deity?
Source: The Great Catechism (New Advent)