Objection 1. It would seem that pride is not the most grievous of sins. For the more difficult a sin is to avoid, the less grievous it would seem to be. Now pride is most difficult to avoid; for Augustine says in his Rule (Ep. ccxi), "Other sins find their vent in the accomplishment of evil deeds, whereas pride lies in wait for good deeds to destroy them." Therefore pride is not the most grievous of sins.
Objection 2. Further, "The greater evil is opposed to the greater good," as the Philosopher asserts (Ethic. viii, 10). Now humility to which pride is opposed is not the greatest of virtues, as stated above (Question 61, Article 5). Therefore the vices that are opposed to greater virtues, such as unbelief, despair, hatred of God, murder, and so forth, are more grievous sins than pride.
Objection 3. Further, the greater evil is not punished by a lesser evil. But pride is sometimes punished by other sins according to Romans 1:28, where it is stated that on account of their pride of heart, men of science were delivered "to a reprobate sense, to do those things which are not convenient." Therefore pride is not the most grievous of sins.
On the contrary, A gloss on Psalm 118:51, "The proud did iniquitously," says: "The greatest sin in man is pride."
I answer that, Two things are to be observed in sin, conversion to a mutable good, and this is the material part of sin; and aversion from the immutable good, and this gives sin its formal aspect and complement. Now on the part of the conversion, there is no reason for pride being the greatest of sins, because uplifting which pride covets inordinately, is not essentially most incompatible with the good of virtue. But on the part of the aversion, pride has extreme gravity, because in other sins man turns away from God, either through ignorance or through weakness, or through desire for any other good whatever; whereas pride denotes aversion from God simply through being unwilling to be subject to God and His rule. Hence Boethius [Cf. Cassian, de Caenob. Onst. xii, 7 says that "while all vices flee from God, pride alone withstands God"; for which reason it is specially stated (James 4:6) that "God resisteth the proud." Wherefore aversion from God and His commandments, which is a consequence as it were in other sins, belongs to pride by its very nature, for its act is the contempt of God. And since that which belongs to a thing by its nature is always of greater weight than that which belongs to it through something else, it follows that pride is the most grievous of sins by its genus, because it exceeds in aversion which is the formal complement of sin.
Reply to Objection 1. A sin is difficult to avoid in two ways. First, on account of the violence of its onslaught; thus anger is violent in its onslaught on account of its impetuosity; and "still more difficult is it to resist concupiscence, on account of its connaturality," as stated in Ethic. ii, 3,9. A difficulty of this kind in avoiding sin diminishes the gravity of the sin; because a man sins the more grievously, according as he yields to a less impetuous temptation, as Augustine says (De Civ. Dei xiv, 12,15).
Secondly, it is difficult to avoid a sin, on account of its being hidden. On this way it is difficult to avoid pride, since it takes occasion even from good deeds, as stated (5, ad 3). Hence Augustine says pointedly that it "lies in wait for good deeds"; and it is written (Psalm 141:4): "In the way wherein I walked, the proud [Cf. Psalm 139:6, 'The proud have hidden a net for me.'] [Vulgate: 'they'] have hidden a snare for me." Hence no very great gravity attaches to the movement of pride while creeping in secretly, and before it is discovered by the judgment of reason: but once discovered by reason, it is easily avoided, both by considering one's own infirmity, according to Sirach 10:9, "Why is earth and ashes proud?" and by considering God's greatness, according to Job 15:13, "Why doth thy spirit swell against God?" as well as by considering the imperfection of the goods on which man prides himself, according to Isaiah 40:6, "All flesh is grass, and all the glory thereof as the flower of the field"; and farther on (Isaiah 64:6), "all our justices" are become "like the rag of a menstruous woman."
Reply to Objection 2. Opposition between a vice and a virtue is inferred from the object, which is considered on the part of conversion. On this way pride has no claim to be the greatest of sins, as neither has humility to be the greatest of virtues. But it is the greatest on the part of aversion, since it brings greatness upon other sins. For unbelief, by the very fact of its arising out of proud contempt, is rendered more grievous than if it be the outcome of ignorance or weakness. The same applies to despair and the like.
Reply to Objection 3. Just as in syllogisms that lead to an impossible conclusion one is sometimes convinced by being faced with a more evident absurdity, so too, in order to overcome their pride, God punishes certain men by allowing them to fall into sins of the flesh, which though they be less grievous are more evidently shameful. Hence Isidore says (De Summo Bono ii, 38) that "pride is the worst of all vices; whether because it is appropriate to those who are of highest and foremost rank, or because it originates from just and virtuous deeds, so that its guilt is less perceptible. on the other hand, carnal lust is apparent to all, because from the outset it is of a shameful nature: and yet, under God's dispensation, it is less grievous than pride. For he who is in the clutches of pride and feels it not, falls into the lusts of the flesh, that being thus humbled he may rise from his abasement."
From this indeed the gravity of pride is made manifest. For just as a wise physician, in order to cure a worse disease, allows the patient to contract one that is less dangerous, so the sin of pride is shown to be more grievous by the very fact that, as a remedy, God allows men to fall into other sins.
Source: Summa Theologica (New Advent)