Thomas Aquinas

“ I answer that, To speak so as to belittle oneself may occur in two ways. First so as to safeguard truth, as when a man conceals the greater things in himself, but discovers and asserts lesser things of himself the presence of which in himself he perceives. ”

“ It belongs to a well-disposed mind that a man tend to perfect righteousness, and consequently deem himself guilty, not only if he fall short of common righteousness, which is truly a sin, but also if he fall short of perfect righteousness, which sometimes is not a sin. But he does not call sinful that which he does not acknowledge to be sinful: which would be a lie of irony. ”

“ Excellence is twofold: one is in temporal, the other in spiritual things. Now it happens at times that a person, by outward words or signs, pretends to be lacking in external things, for instance by wearing shabby clothes, or by doing something of the kind, and that he intends by so doing to make a show of some spiritual excellence. Thus our Lord said of certain men (Mat. 6:16) that "they disfigure their faces that they may appear unto men to fast." Wherefore such persons are guilty of both vices, irony and boasting, although in different respects, and for this reason they sin more grievously. ”