He came in human rather than in any nobler form, because (I) He came to save, not to impress ; (2) man alone of creatures had sinned. As men would not recognise His works in the Universe, He came and worked among them as Man; in the sphere to which they had limited themselves
Now, if they ask, Why then did He not appear by means of other and nobler parts of creation, and use some nobler instrument, as the sun, or moon, or stars, or fire, or air, instead of man merely? Let them know that the Lord came not to make a display, but to heal and teach those who were suffering. 2. For the way for one aiming at display would be, just to appear, and to dazzle the beholders; but for one seeking to heal and teach the way is, not simply to sojourn here, but to give himself to the aid of those in want, and to appear as they who need him can bear it; that he may not, by exceeding the requirements of the sufferers, trouble the very persons that need him, rendering God's appearance useless to them. 3.
Now, nothing in creation had gone astray with regard to their notions of God, save man only. Why, neither sun, nor moon, nor heaven, nor the stars, nor water, nor air had swerved from their order; but knowing their Artificer and Sovereign, the Word, they remain as they were made. But men alone, having rejected what was good, then devised things of nought instead of the truth, and have ascribed the honour due to God, and their knowledge of Him, to demons and men in the shape of stones. 4.
With reason, then, since it were unworthy of the Divine Goodness to overlook so grave a matter, while yet men were not able to recognise Him as ordering and guiding the whole, He takes to Himself as an instrument a part of the whole, His human body, and unites Himself with that, in order that since men could not recognise Him in the whole, they should not fail to know Him in the part; and since they could not look up to His invisible power, might be able, at any rate, from what resembled themselves to reason to Him and to contemplate Him. 5.
For, men as they are, they will be able to know His Father more quickly and directly by a body of like nature and by the divine works wrought through it, judging by comparison that they are not human, but the works of God, which are done by Him. 6. And if it were absurd, as they say, for the Word to be known through the works of the body, it would likewise be absurd for Him to be known through the works of the universe. For just as He is in creation, and yet does not partake of its nature in the least degree, but rather all things partake of His power; so while He used the body as His instrument He partook of no corporeal property, but, on the contrary, Himself sanctified even the body. 7.
For if even Plato, who is in such repute among the Greeks, says that its author, beholding the universe tempest-tossed, and in peril of going down to the place of chaos, takes his seat at the helm of the soul and comes to the rescue and corrects all its calamities; what is there incredible in what we say, that, mankind being in error, the Word lighted down upon it and appeared as man, that He might save it in its tempest by His guidance and goodness?
Source: On the Incarnation of the Word (New Advent)