From today’s (Ash Wednesday’s) Matins reading (in the 1960 edition):
Augustine’s commentary on today’s Gospel: And when you fast, be not as the hypocrites, sad. For they disfigure their faces, that they may appear unto men to fast. Amen I say to you, they have received their reward.
My own translation – a little freer than I usually like, but the only way to produce a good English version of the Latin.
From these commands it is obvious that all our focus should be directed towards interior joys, lest by seeking outside for our reward we conform to this world and lose the promise of happiness – which is more stable and firm the more interior it is – by which God chose us to become conformed to the image of his Son.
But in this passage we should especially notice that one can be ostentatious not only in the splendid display of material things, but even in mournful drabness itself; and that is more dangerous, for it deceives in the guise of servitude to God.
So the one who shows off in excessively cultivating his body and in splendid clothing or other things is easily convicted as a devotee of the world by those same ostentatious things, and he does not mislead anyone with his fake image of holiness.
But the one who, in professing his Christianity, makes others’ eyes focus on himself by his unaccustomed squalor and mournfulness – when he does it by will, not undergoing it out of necessity – he can be appraised by his other deeds: is he doing this out of contempt for gratuitous cultivation, or out of some kind of ambition?
For the Lord commanded us to beware wolves in sheep’s clothing. By their fruits you shall know them, he said.
So when the very things which, under this disguise, he either is pursuing or wants to pursue start to be taken away or denied him by some kind of trial, then it will inevitably appear whether he is a wolf in sheep’s clothing or a sheep in its own.
For if it is true that pretenders often assume a spare and minimal attire so as to deceive the gullible, a Christian should not on that account entice others’ attention with gratuitous adornments. The sheep should not abandon their native clothing just because wolves sometimes dress themselves in it.
(St. Augustine, The Lord’s Sermon on the Mount II.12.40-41)