Objection 1. It would seem that pride is not the first sin of all. For the first is maintained in all that follows. Now pride does not accompany all sins, nor is it the origin of all: for Augustine says (De Nat. et Grat. xx) that many things are done "amiss which are not done with pride." Therefore pride is not the first sin of all.
Objection 2. Further, it is written (Sirach 10:14) that the "beginning of... pride is to fall off from God." Therefore falling away from God precedes pride.
Objection 3. Further, the order of sins would seem to be according to the order of virtues. Now, not humility but faith is the first of all virtues. Therefore pride is not the first sin of all.
Objection 4. Further, it is written (2 Timothy 3:13): "Evil men and seducers shall grow worse and worse"; so that apparently man's beginning of wickedness is not the greatest of sins. But pride is the greatest of sins as stated in the foregoing Article. Therefore pride is not the first sin.
Objection 5. Further, resemblance and pretense come after the reality. Now the Philosopher says (Ethic. iii, 7) that "pride apes fortitude and daring." Therefore the vice of daring precedes the vice of pride.
On the contrary, It is written (Sirach 10:15): "Pride is the beginning of all sin."
I answer that, The first thing in every genus is that which is essential. Now it has been stated above (Article 6) that aversion from God, which is the formal complement of sin, belongs to pride essentially, and to other sins, consequently. Hence it is that pride fulfils the conditions of a first thing, and is "the beginning of all sins," as stated above (I-II, 84, 2), when we were treating of the causes of sin on the part of the aversion which is the chief part of sin.
Reply to Objection 1. Pride is said to be "the beginning of all sin," not as though every sin originated from pride, but because any kind of sin is naturally liable to arise from pride.
Reply to Objection 2. To fall off from God is said to be the beginning of pride, not as though it were a distinct sin from pride, but as being the first part of pride. For it has been said above (Article 5) that pride regards chiefly subjection to God which it scorns, and in consequence it scorns to be subject to a creature for God's sake.
Reply to Objection 3. There is no need for the order of virtues to be the same as that of vices. For vice is corruptive of virtue. Now that which is first to be generated is the last to be corrupted. Wherefore as faith is the first of virtues, so unbelief is the last of sins, to which sometimes man is led by other sins. Hence a gloss on Psalm 136:7, "Rase it, rase it, even to the foundation thereof," says that "by heaping vice upon vice a man will lapse into unbelief," and the Apostle says (1 Timothy 1:19) that "some rejecting a good conscience have made shipwreck concerning the faith."
Reply to Objection 4. Pride is said to be the most grievous of sins because that which gives sin its gravity is essential to pride. Hence pride is the cause of gravity in other sins. Accordingly previous to pride there may be certain less grievous sins that are committed through ignorance or weakness. But among the grievous sins the first is pride, as the cause whereby other sins are rendered more grievous. And as that which is the first in causing sins is the last in the withdrawal from sin, a gloss on Psalm 18:13, "I shall be cleansed from the greatest sin," says: "Namely from the sin of pride, which is the last in those who return to God, and the first in those who withdraw from God."
Reply to Objection 5. The Philosopher associates pride with feigned fortitude, not that it consists precisely in this, but because man thinks he is more likely to be uplifted before men, if he seem to be daring or brave.
Source: Summa Theologica (New Advent)