Boastfulness

150. Boastfulness is a vice by which man, desiring to be supremely honoured above all others, begins to praise and exalt himself, exaggerating and amplifying things so as to make his own merit appear greater than it is. It is also called ostentation, self-praise or forwardness; and St. Augustine calls it "The worst of all pests"; [Lib. 1 De Ord. cap. xi] and St. Ambrose calls it a net spread by the devil to catch the strongest and most spiritual: "The devil lays snares such as entrap the strongest"; [Lib. in Luc.] and this is a vice which is beyond measure, because in vaunting ourselves for that which we have not, we lie to our own conscience and to God; and as God said of Moab by the prophet: "He is exceeding proud; I know his boasting, and that the strength thereof is not according to it." [Jer. xlviii, 29, 30]
It can be a mortal sin when we boast of some sin which we have committed; when we praise ourselves, despising others; or else when we praise and exalt ourselves through an excess of pride which abounds in the heart.
The Angelical Doctor notes that this is an ordinary and not an infrequent case, and that the habit is easily formed. [2a 2æ, qu. lxii, art. 1. See also 2a 2æ, qu. cx, art. 2; qu. cxii, art. 1; et qu. cxxxii, art. 5 ad 1; et qu. clxii, art. 4 ad 2]