Objection 1. It would seem that, in the Law, there should not have been given a precept of fear. For the fear of God is about things which are a preamble to the Law, since it is the "beginning of wisdom." Now things which are a preamble to the Law do not come under a precept of the Law. Therefore no precept of fear should be given in the Law.
Objection 2. Further, given the cause, the effect is also given. Now love is the cause of fear, since "every fear proceeds from some kind of love," as Augustine states (Qq. lxxxiii, qu. 33). Therefore given the precept of love, it would have been superfluous to command fear.
Objection 3. Further, presumption, in a way, is opposed to fear. But the Law contains no prohibition against presumption. Therefore it seems that neither should any precept of fear have been given.
On the contrary, It is written (Deuteronomy 10:12): "And now, Israel, what doth the Lord thy God require of thee, but that thou fear the Lord thy God?"
But He requires of us that which He commands us to do. Therefore it is a matter of precept that man should fear God.
I answer that, Fear is twofold, servile and filial. Now just as man is induced, by the hope of rewards, to observe precepts of law, so too is he induced thereto by the fear of punishment, which fear is servile.
And just as according to what has been said (1), in the promulgation of the Law there was no need for a precept of the act of hope, and men were to be induced thereto by promises, so neither was there need for a precept, under form of command, of fear which regards punishment, and men were to be induced thereto by the threat of punishment: and this was realized both in the precepts of the decalogue, and afterwards, in due sequence, in the secondary precepts of the Law.
Yet, just as wise men and the prophets who, consequently, strove to strengthen man in the observance of the Law, delivered their teaching about hope under the form of admonition or command, so too did they in the matter of fear.
On the other hand filial fear which shows reverence to God, is a sort of genus in respect of the love of God, and a kind of principle of all observances connected with reverence for God. Hence precepts of filial fear are given in the Law, even as precepts of love, because each is a preamble to the external acts prescribed by the Law and to which the precepts of the decalogue refer. Hence in the passage quoted in the argument, "On the contrary," man is required "to have fear, to walk in God's ways," by worshipping Him, and "to love Him."
Reply to Objection 1. Filial fear is a preamble to the Law, not as though it were extrinsic thereto, but as being the beginning of the Law, just as love is. Hence precepts are given of both, since they are like general principles of the whole Law.
Reply to Objection 2. From love proceeds filial fear as also other good works that are done from charity. Hence, just as after the precept of charity, precepts are given of the other acts of virtue, so at the same time precepts are given of fear and of the love of charity, just as, in demonstrative sciences, it is not enough to lay down the first principles, unless the conclusions also are given which follow from them proximately or remotely.
Reply to Objection 3. Inducement to fear suffices to exclude presumption, even as inducement to hope suffices to exclude despair, as stated above (1, ad 3).
Source: Summa Theologica (New Advent)