But not to mention all arguments, let us bring forward that one to which all arguments point, that, for one who was God to assume a lowly guise both has an obvious use, and is an adaptation and in nothing contradicts the course of nature. But for one who is man to speak things divine and supernatural is the highest presumption; for though a king may humble himself a common soldier may not take on him the state of an emperor. So, if He were God made man, all lowly things have place; but if mere man, high things have none.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Epist. lib. iv. 166