HKB
Reply Obj. 1: This argument takes boasting as exceeding men's opinion.
Reply Obj. 2: The sin of boasting may be considered in two ways. First, with regard to the species of the act, and thus it is opposed to truth; as stated (in the body of the article and Q. 110, A. 2). Secondly, with regard to its cause, from which more frequently though not always it arises: and thus it proceeds from pride as its inwardly moving and impelling cause. For when a man is uplifted inwardly by arrogance, it often results that outwardly he boasts of great things about himself; though sometimes a man takes to boasting, not from arrogance, but from some kind of vanity, and delights therein, because he is a boaster by habit. Hence arrogance, which is an uplifting of self above oneself, is a kind of pride; yet it is not the same as boasting, but is very often its cause. For this reason Gregory reckons boasting among the species of pride. Moreover, the boaster frequently aims at obtaining glory through his boasting, and so, according to Gregory, it arises from vainglory considered as its end.
Reply Obj. 3: Wealth also causes boasting, in two ways. First, as an occasional cause, inasmuch as a man prides himself on his riches. Hence (Prov. 8:18) "riches" are significantly described as "proud" [Douay: 'glorious']. Secondly, as being the end of boasting, since according to Ethic. iv, 7, some boast, not only for the sake of glory, but also for the sake of gain. Such people invent stories about themselves, so as to make profit thereby; for instance, they pretend to be skilled in medicine, wisdom, or divination.
SECOND ARTICLE [II-II, Q. 112, Art. 2]
Whether Boasting Is a Mortal Sin?
Objection 1: It seems that boasting is a mortal sin. For it is written (Prov. 28:25): "He that boasteth, and puffeth himself, stirreth up quarrels." Now it is a mortal sin to stir up quarrels, since God hates those that sow discord, according to Prov. 6:19. Therefore boasting is a mortal sin.
Obj. 2: Further, whatever is forbidden in God's law is a mortal sin. Now a gloss on Ecclus. 6:2, "Extol not thyself in the thoughts of thy soul," says: "This is a prohibition of boasting and pride." Therefore boasting is a mortal sin.
Obj. 3: Further, boasting is a kind of lie. But it is neither an officious nor a jocose lie. This is evident from the end of lying; for according to the Philosopher (Ethic. iv, 7), "the boaster pretends to something greater than he is, sometimes for no further purpose, sometimes for the sake of glory or honor, sometimes for the sake of money." Thus it is evident that it is neither an officious nor a jocose lie, and consequently it must be a mischievous lie. Therefore seemingly it is always a mortal sin.
On the contrary, Boasting arises from vainglory, according to Gregory (Moral. xxxi, 17). Now vainglory is not always a mortal sin, but is sometimes a venial sin which only the very perfect avoid. For Gregory says (Moral. viii, 30) that "it belongs to the very perfect, by outward deeds so to seek the glory of their author, that they are not inwardly uplifted by the praise awarded them." Therefore boasting is not always a mortal sin.