Objection 1. It would seem that pride is not a mortal sin. For a gloss on Psalm 7:4, "O Lord my God, if I have done this thing," says: "Namely, the universal sin which is pride." Therefore if pride were a mortal sin, so would every sin be.
Objection 2. Further, every mortal sin is contrary to charity. But pride is apparently not contrary to charity, neither as to the love of God, nor as to the love of one's neighbor, because the excellence which, by pride, one desires inordinately, is not always opposed to God's honor, or our neighbor's good. Therefore pride is not a mortal sin.
Objection 3. Further, every mortal sin is opposed to virtue. But pride is not opposed to virtue; on the contrary, it arises therefrom, for as Gregory says (Moral. xxxiv, 23), "sometimes a man is elated by sublime and heavenly virtues." Therefore pride is not a mortal sin.
On the contrary, Gregory says (Moral. xxxiv, 23) that "pride is a most evident sign of the reprobate, and contrariwise, humility of the elect." But men do not become reprobate on account of venial sins. Therefore pride is not a venial but a mortal sin.
I answer that, Pride is opposed to humility. Now humility properly regards the subjection of man to God, as stated above (161, 1, ad 5). Hence pride properly regards lack of this subjection, in so far as a man raises himself above that which is appointed to him according to the Divine rule or measure, against the saying of the Apostle (2 Corinthians 10:13), "But we will not glory beyond our measure; but according to the measure of the rule which God hath measured to us." Wherefore it is written (Sirach 10:14): "The beginning of the pride of man is to fall off from God" because, to wit, the root of pride is found to consist in man not being, in some way, subject to God and His rule. Now it is evident that not to be subject to God is of its very nature a mortal sin, for this consists in turning away from God: and consequently pride is, of its genus, a mortal sin. Nevertheless just as in other sins which are mortal by their genus (for instance fornication and adultery) there are certain motions that are venial by reason of their imperfection (through forestalling the judgment of reason, and being without its consent), so too in the matter of pride it happens that certain motions of pride are venial sins, when reason does not consent to them.
Reply to Objection 1. As stated above (Article 2) pride is a general sin, not by its essence but by a kind of influence, in so far as all sins may have their origin in pride. Hence it does not follow that all sins are mortal, but only such as arise from perfect pride, which we have stated to be a mortal sin.
Reply to Objection 2. Pride is always contrary to the love of God, inasmuch as the proud man does not subject himself to the Divine rule as he ought. Sometimes it is also contrary to the love of our neighbor; when, namely, a man sets himself inordinately above his neighbor: and this again is a transgression of the Divine rule, which has established order among men, so that one ought to be subject to another.
Reply to Objection 3. Pride arises from virtue, not as from its direct cause, but as from an accidental cause, in so far as a man makes a virtue an occasion for pride. And nothing prevents one contrary from being the accidental cause of another, as stated in Phys. viii, 1. Hence some are even proud of their humility.
Source: Summa Theologica (New Advent)