Objection 1. It would seem that pride is not the beginning of every sin. For the root is a beginning of a tree, so that the beginning of a sin seems to be the same as the root of sin. Now covetousness is the root of every sin, as stated above (Article 1). Therefore it is also the beginning of every sin, and not pride.
Objection 2. Further, it is written (Sirach 10:14): "The beginning of the pride of man is apostasy [Douay: 'to fall off'] from God." But apostasy from God is a sin. Therefore another sin is the beginning of pride, so that the latter is not the beginning of every sin.
Objection 3. Further, the beginning of every sin would seem to be that which causes all sins. Now this is inordinate self-love, which, according to Augustine (De Civ. Dei xiv), "builds up the city of Babylon." Therefore self-love and not pride, is the beginning of every sin.
On the contrary, It is written (Sirach 10:15): "Pride is the beginning of all sin."
I answer that, Some say pride is to be taken in three ways. First, as denoting inordinate desire to excel; and thus it is a special sin. Secondly, as denoting actual contempt of God, to the effect of not being subject to His commandment; and thus, they say, it is a generic sin. Thirdly, as denoting an inclination to this contempt, owing to the corruption of nature; and in this sense they say that it is the beginning of every sin, and that it differs from covetousness, because covetousness regards sin as turning towards the mutable good by which sin is, as it were, nourished and fostered, for which reason covetousness is called the "root"; whereas pride regards sin as turning away from God, to Whose commandment man refuses to be subject, for which reason it is called the "beginning," because the beginning of evil consists in turning away from God.
Now though all this is true, nevertheless it does not explain the mind of the wise man who said (Sirach 10:15): "Pride is the beginning of all sin." For it is evident that he is speaking of pride as denoting inordinate desire to excel, as is clear from what follows (verse 17): "God hath overturned the thrones of proud princes"; indeed this is the point of nearly the whole chapter. We must therefore say that pride, even as denoting a special sin, is the beginning of every sin. For we must take note that, in voluntary actions, such as sins, there is a twofold order, of intention, and of execution. In the former order, the principle is the end, as we have stated many times before (1, 1, ad 1; 18, 7, ad 2; 15, 1, ad 2; 25, 2). Now man's end in acquiring all temporal goods is that, through their means, he may have some perfection and excellence. Therefore, from this point of view, pride, which is the desire to excel, is said to be the "beginning" of every sin. On the other hand, in the order of execution, the first place belongs to that which by furnishing the opportunity of fulfilling all desires of sin, has the character of a root, and such are riches; so that, from this point of view, covetousness is said to be the "root" of all evils, as stated above (Article 1).
This suffices for the Reply to the First Objection.
Reply to Objection 2. Apostasy from God is stated to be the beginning of pride, in so far as it denotes a turning away from God, because from the fact that man wishes not to be subject to God, it follows that he desires inordinately his own excellence in temporal things. Wherefore, in the passage quoted, apostasy from God does not denote the special sin, but rather that general condition of every sin, consisting in its turning away from God. It may also be said that apostasy from God is said to be the beginning of pride, because it is the first species of pride. For it is characteristic of pride to be unwilling to be subject to any superior, and especially to God; the result being that a man is unduly lifted up, in respect of the other species of pride.
Reply to Objection 3. In desiring to excel, man loves himself, for to love oneself is the same as to desire some good for oneself. Consequently it amounts to the same whether we reckon pride or self-love as the beginning of every evil.
Source: Summa Theologica (New Advent)