9 The fact that He took rest in sleep, and was nourished by food, and experienced all the feelings of humanity, is the evidence to men of the reality of that human nature which He assumed but did not destroy. Behold, this was the fact; and yet some heretics, by a perverted admiration and praise of His power, have refused altogether to acknowledge the reality of His human nature, in which is the guarantee of all that grace by which He saves those who believe in Him, containing deep treasures of wisdom and knowledge, and imparting faith to the minds which He raises to the eternal contemplation of unchangeable truth. What if the Almighty had created the human nature of Christ not by causing Him to be born of a mother, but by some other way, and had presented Him suddenly to the eyes of mankind? What if the Lord had not passed through the stages of progress from infancy to manhood, and had taken neither food nor sleep? Would not this have confirmed the erroneous impression above referred to, and have made it impossible to believe at all that He had taken to Himself true human nature; and, while leaving what was marvellous, would eliminate the element of mercy from His actions? But now He has so appeared as the Mediator between God and men, that, uniting the two natures in one person, He both exalted what was ordinary by what was extraordinary, and tempered what was extraordinary by what was ordinary in Himself.
10. But where in all the varied movements of creation is there any work of God which is not wonderful, were it not that through familiarity these wonders have become small in our esteem? Nay, how many common things are trodden under foot, which, if examined carefully, awaken our astonishment! Take, for example, the propertries of seeds: who can either comprehend or declare the variety of species, the vitality, vigour, and secret power by which they from within small compass evolve great things? Now the human body and soul which He took to Himself was created without seed by Him who in the natural world created originally seeds from no pre-existent seeds. In the body which thus became His, he who, without any liability to change in Himself, has woven according to His counsel the vicissitudes of all past centuries, became subject to the succession of seasons and the ordinary stages of the life of man. For His body, as it began to exist at a point of time, became developed with the lapse of time. But the Word of God, who was in the beginning, and to whom the ages of time owe their existence, did not bow to time as bringing round the event of His incarnation apart from His consent, but chose the point of time at which He freely took our nature to Himself. The human nature was brought into union with the divine; God did not withdraw from Himself.
11. Some resist upon being furnished with an explanation of the manner in which the Godhead was so united with a human soul and body as to constitute the one person of Christ, when it was necessary that this should be done once in the world's history, with as much boldness as if they were themselves able to furnish an explanation of the manner in which the soul is so united to the body as to constitute the one person of man, an event which is occurring every day. For just as the soul is united to the body in one person so as to constitute man, in the same way God united to man in one person so as to constitute Christ. In the former personality there is a combination of soul and body; in the latter there is a combination of the Godhead and man. Let my reader, however, guard against borrowing his idea of the combination from the properties of material bodies, by which two fluids when combined are so mixed that neither preserves its original character; although even among material bodies there are exceptions, such as light, which sustains no change when combined with the atmosphere. In the person of man, therefore, there is a combination of soul and body; in the person of Christ there is a combination of the Godhead with man; for when the Word of God was united to a soul having a body, He took into union with Himself both the soul and the body. The former event takes place daily in the beginning of life in individuals of the human race; the latter took place once for the salvation of men. And yet of the two events, the combination of two immaterial substances ought to be more easily believed than a combination in which the one is immaterial and the other material. For if the soul is not mistaken in regard to its own nature, it understands itself to be immaterial. Much more certainly does this attribute belong to the Word of God; and consequently the combination of the Word with the human soul is a combination which ought to be much more credible than that of soul and body. The latter is realized by us in ourselves; the former we are commanded to believe to have been realized in Christ. But if both of them were alike foreign to our experience, and we were enjoined to believe that both had taken place, which of the two would we more readily believe to have occurred? Would we not admit that two immaterial substances could be more easily combined than one immaterial and one material; unless, perhaps, it be unsuitable to use the word combination in connection with these things, because of the difference between their nature and that of material substances, both in themselves and as known to us?
12. Wherefore the Word of God, who is also the Son of God, co-eternal with the Father, the Power and the Wisdom of God, mightily pervading and harmoniously ordering all things, from the highest limit of the intelligent to the lowest limit of the material creation, revealed and concealed, nowhere confined, nowhere divided, nowhere distended, but without dimensions, everywhere present in His entirety—this Word of God, I say, took to Himself, in a manner entirely different from that in which He is present to other creatures, the soul and body of a man, and made, by the union of Himself therewith, the one person Jesus Christ, Mediator between God and men, in His Deity equal with the Father, in His flesh, i.e. in His human nature, inferior to the Father—unchangeably immortal in respect of the divine nature, in which He is equal with the Father, and yet changeable and mortal in respect of the infirmity which was His through participation with our nature.
Source: Letters (New Advent)