The Divine Institutes

It remains to answer those also, who deem that it was unbecoming and unreasonable that God should be clothed with a mortal body; that He should be in subjection to men; that He should endure insults; that He should even suffer tortures and death. I will speak my sentiments, and I will sum up, as I shall be able, an immense subject in few words. He who teaches anything, ought, as I think, himself to practice what he teaches, that he may compel men to obey. For if he shall not practice them, he will detract from the faith due to his precepts. Therefore there is need of examples, that the precepts which are given may have firmness, and if any one shall prove contumacious, and shall say that they cannot be carried out in practice, the instructor may refute him by actual fact. Therefore a system of teaching cannot be perfect, when it is delivered by words only; but it then becomes perfect, when it is completed by deeds.

Since therefore Christ was sent to men as a teacher of virtue, for the perfection of His teaching it was plainly befitting that He should act as well as teach. But if He had not assumed a human body, He would not have been able to practice what He taught,—that is, not to be angry, not to desire riches, not to be inflamed with lust, not to fear pain, to despise death. These things are plainly virtues, but they cannot be done without flesh. Therefore He assumed a body on this account, that, since He taught that the desires of the flesh must be overcome, He might in person first practice it, that no one might allege the frailty of the flesh as an excuse.