Objection 1. It would seem unlawful for religious to wear coarser clothes than others. For according to the Apostle (1 Thessalonians 5:2)2) we ought to "refrain from all appearance of evil." Now coarseness of clothes has an appearance of evil; for our Lord said (Matthew 7:15): "Beware of false prophets who come to you in the clothing of sheep": and a gloss on Apocalypse 6:8, "Behold a pale horse," says: "The devil finding that he cannot succeed, neither by outward afflictions nor by manifest heresies, sends in advance false brethren, who under the guise of religion assume the characteristics of the black and red horses by corrupting the faith." Therefore it would seem that religious should not wear coarse clothes.
Objection 2. Further, Jerome says (Ep. lii ad Nepotian.): "Avoid somber," i.e. black, "equally with glittering apparel. Fine and coarse clothes are equally to be shunned, for the one exhales pleasure, the other vainglory." Therefore, since vainglory is a graver sin than the use of pleasure, it would seem that religious who should aim at what is more perfect ought to avoid coarse rather than fine clothes.
Objection 3. Further, religious should aim especially at doing works of penance. Now in works of penance we should use, not outward signs of sorrow, but rather signs of joy; for our Lord said (Matthew 6:16): "When you fast, be not, as the hypocrites, sad," and afterwards He added: "But thou, when thou fastest, anoint thy head and wash thy face." Augustine commenting on these words (De Serm. Dom. in Monte ii, 12): "In this chapter we must observe that not only the glare and pomp of outward things, but even the weeds of mourning may be a subject of ostentation, all the more dangerous as being a decoy under the guise of God's service." Therefore seemingly religious ought not to wear coarse clothes.
On the contrary, The Apostle says (Hebrews 11:3)7): "They wandered about in sheep-skins in goat-skins," and a gloss adds--"as Elias and others." Moreover it is said in the Decretal XXI, qu. iv, can. Omnis jactantia: "If any persons be found to deride those who wear coarse and religious apparel they must be reproved. For in the early times all those who were consecrated to God went about in common and coarse apparel."
I answer that, As Augustine says (De Doctr. Christ. iii, 12), "in all external things, it is not the use but the intention of the user that is at fault." On order to judge of this it is necessary to observe that coarse and homely apparel may be considered in two ways. First, as being a sign of a man's disposition or condition, because according to Sirach 19:27, "the attire... of the man" shows "what he is." On this way coarseness of attire is sometimes a sign of sorrow: wherefore those who are beset with sorrow are wont to wear coarser clothes, just as on the other hand in times of festivity and joy they wear finer clothes. Hence penitents make use of coarse apparel, for example, the king (Jonah 3:6) who "was clothed with sack-cloth," and Achab (1 Kings 21:27) who "put hair-cloth upon his flesh." Sometimes, however, it is a sign of the contempt of riches and worldly ostentation. Wherefore Jerome says (Ep. cxxv ad Rustico Monach.): "Let your somber attire indicate your purity of mind, your coarse robe prove your contempt of the world, yet so that your mind be not inflated withal, lest your speech belie your habit." On both these ways it is becoming for religious to wear coarse attire, since religion is a state of penance and of contempt of worldly glory.
But that a person wish to signify this to others arises from three motives. First, in order to humble himself: for just as a man's mind is uplifted by fine clothes, so is it humbled by lowly apparel. Hence speaking of Achab who "put hair-cloth on his flesh," the Lord said to Elias: "Hast thou not seen Achab humbled before Me?" (1 Kings 21:29). Secondly, in order to set an example to others; wherefore a gloss on Matthew 3:4, "(John) had his garments of camel's hair," says: "He who preaches penance is clothed in the habit of penance." Thirdly, on account of vainglory; thus Augustine says (cf. Objection 3) that "even the weeds of mourning may be a subject of ostentation."
Accordingly in the first two ways it is praiseworthy to wear humble apparel, but in the third way it is sinful.
Secondly, coarse and homely attire may be considered as the result of covetousness or negligence, and thus also it is sinful.
Reply to Objection 1. Coarseness of attire has not of itself the appearance of evil, indeed it has more the appearance of good, namely of the contempt of worldly glory. Hence it is that wicked persons hide their wickedness under coarse clothing. Hence Augustine says (De Serm. Dom. in Monte ii, 24) that "the sheep should not dislike their clothing for the reason that the wolves sometimes hide themselves under it."
Reply to Objection 2. Jerome is speaking there of the coarse attire that is worn on account of human glory.
Reply to Objection 3. According to our Lord's teaching men should do no deeds of holiness for the sake of show: and this is especially the case when one does something strange. Hence Chrysostom [Hom. xiii in Matth. in the Opus Imperfectum, falsely ascribed to St. John Chrysostom] says: "While praying a man should do nothing strange, so as to draw the gaze of others, either by shouting or striking his breast, or casting up his hands," because the very strangeness draws people's attention to him. Yet blame does not attach to all strange behavior that draws people's attention, for it may be done well or ill. Hence Augustine says (De Serm. Dom. in Monte ii, 12) that "in the practice of the Christian religion when a man draws attention to himself by unwonted squalor and shabbiness, since he acts thus voluntarily and not of necessity, we can gather from his other deeds whether his behavior is motivated by contempt of excessive dress or by affectation." Religious, however, would especially seem not to act thus from affectation, since they wear a coarse habit as a sign of their profession whereby they profess contempt of the world.
Source: Summa Theologica (New Advent)