Objection 1. It would seem that fear is not the beginning of wisdom. For the beginning of a thing is a part thereof. But fear is not a part of wisdom, since fear is seated in the appetitive faculty, while wisdom is in the intellect. Therefore it seems that fear is not the beginning of wisdom.
Objection 2. Further, nothing is the beginning of itself. "Now fear of the Lord, that is wisdom," according to Job 28:28. Therefore it seems that fear of God is not the beginning of wisdom.
Objection 3. Further, nothing is prior to the beginning. But something is prior to fear, since faith precedes fear. Therefore it seems that fear is not the beginning of wisdom.
On the contrary, It is written in the Psalm 110:10: "The fear of the Lord is the beginning of wisdom."
I answer that, A thing may be called the beginning of wisdom in two ways: in one way because it is the beginning of wisdom itself as to its essence; in another way, as to its effect. Thus the beginning of an art as to its essence consists in the principles from which that art proceeds, while the beginning of an art as to its effect is that wherefrom it begins to operate: for instance we might say that the beginning of the art of building is the foundation because that is where the builder begins his work.
Now, since wisdom is the knowledge of Divine things, as we shall state further on (45, 1), it is considered by us in one way, and in another way by philosophers. For, seeing that our life is ordained to the enjoyment of God, and is directed thereto according to a participation of the Divine Nature, conferred on us through grace, wisdom, as we look at it, is considered not only as being cognizant of God, as it is with the philosophers, but also as directing human conduct; since this is directed not only by the human law, but also by the Divine law, as Augustine shows (De Trin. xii, 14). Accordingly the beginning of wisdom as to its essence consists in the first principles of wisdom, i.e. the articles of faith, and in this sense faith is said to be the beginning of wisdom. But as regards the effect, the beginning of wisdom is the point where wisdom begins to work, and in this way fear is the beginning of wisdom, yet servile fear in one way, and filial fear, in another. For servile fear is like a principle disposing a man to wisdom from without, in so far as he refrains from sin through fear of punishment, and is thus fashioned for the effect of wisdom, according to Sirach 1:27, "The fear of the Lord driveth out sin." On the other hand, chaste or filial fear is the beginning of wisdom, as being the first effect of wisdom. For since the regulation of human conduct by the Divine law belongs to wisdom, in order to make a beginning, man must first of all fear God and submit himself to Him: for the result will be that in all things he will be ruled by God.
Reply to Objection 1. This argument proves that fear is not the beginning of wisdom as to the essence of wisdom.
Reply to Objection 2. The fear of God is compared to a man's whole life that is ruled by God's wisdom, as the root to the tree: hence it is written (Sirach 1:25): "The root of wisdom is to fear the Lord, for [Vulgate: 'and'] the branches thereof are longlived." Consequently, as the root is said to be virtually the tree, so the fear of God is said to be wisdom.
Reply to Objection 3. As stated above, faith is the beginning of wisdom in one way, and fear, in another. Hence it is written (Sirach 25:16): "The fear of God is the beginning of love: and the beginning of faith is to be fast joined to it."
Source: Summa Theologica (New Advent)