The Divine Institutes

Let men therefore learn and understand why the Most High God, when He sent His ambassador and messenger to instruct mortals with the precepts of His righteousness, willed that He should be clothed with mortal flesh, and be afflicted with torture, and be sentenced to death. For since there was no righteousness on earth, He sent a teacher, as it were a living law, to found a new name and temple, that by His words and example He might spread throughout the earth a true and holy worship. But, however, that it might be certain that He was sent by God, it was befitting that He should not be born as man is born, composed of a mortal on both sides; but that it might appear that He was heavenly even in the form of man, He was born without the office of a father. For He had a spiritual Father, God; and as God was the Father of His spirit without a mother, so a virgin was the mother of His body without a father. He was therefore both God and man, being placed in the middle between God and man. From which the Greeks call Him Mesites, that He might be able to lead man to God— that is, to immortality: for if He had been God only (as we have before said), He would not have been able to afford to man examples of goodness; if He had been man only, He would not have been able to compel men to righteousness, unless there had been added an authority and virtue greater than that of man.

For, since man is composed of flesh and spirit, and the spirit must earn immortality by works of righteousness, the flesh, since it is earthly, and therefore mortal, draws with itself the spirit linked to it, and leads it from immortality to death. Therefore the spirit, apart from the flesh, could by no means be a guide to immortality for man, since the flesh hinders the spirit from following God. For it is frail, and liable to sin; but sin is the food and nourishment of death. For this cause, therefore, a mediator came— that is, God in the flesh— that the flesh might be able to follow Him, and that He might rescue man from death, which has dominion over the flesh. Therefore He clothed Himself with flesh, that the desires of the flesh being subdued, He might teach that to sin was not the result of necessity, but of man's purpose and will. For we have one great and principal struggle to maintain with the flesh, the boundless desires of which press upon the soul, nor allow it to retain dominion, but make it the slave of pleasures and sweet allurements, and visit it with everlasting death. And that we might be able to overcome these, God has opened and displayed to us the way of overcoming the flesh. And this perfect and absolutely complete virtue bestows on those who conquer, the crown and reward of immortality.