The benefit of the law – fear.

John Calvin has written:

“10. The second office of the Law is, by means of its fearful denunciations and the consequent dread of punishment, to curb those who, unless forced, have no regard for rectitude and justice. Such persons are curbed, not because their mind is inwardly moved and affected, but because, as if a bridle were laid upon them, they refrain their hands from external acts, and internally check the depravity which would otherwise petulantly burst forth. It is true, they are not on this account either better or more righteous in the sight of God. For although restrained by terror or shame, they dare not proceed to what their mind has conceived, nor give full license to their raging lust, their heart is by no means trained to fear and obedience. Nay, the more they restrain themselves, the more they are inflamed, the more they rage and boil, prepared for any act or outbreak whatsoever, were it not for the terror of the law. And not only so, but they thoroughly detest the law itself, and execrate the Lawgiver; so that if they could, they would most willingly annihilate him, because they cannot bear either his ordering what is right, or his avenging the despisers of his Majesty. The feeling of all who are not yet regenerate, though in some more, in others less lively, is, that in regard to the observance of the law, they are not led by voluntary submission, but dragged by the force of fear. Nevertheless, this forced and extorted righteousness is necessary for the good of society, its peace being secured by a provision but for which all things would be thrown into tumult and confusion. Nay, this tuition is not without its use, even to the children of God, who, previous to their effectual calling, being destitute of the Spirit of holiness, freely indulge the lusts of the flesh. When, by the fear of Divine vengeance, they are deterred from open outbreakings, though, from not being subdued in mind, they profit little at present, still they are in some measure trained to bear the yoke of righteousness, so that when they are called, they are not like mere novices, studying a discipline of which previously they had no knowledge. This office seems to be especially in the view of the Apostle, when he says, “That the law is not made for a righteous man, but for the lawless and disobedient, for the ungodly and for sinners, for unholy and profane, for murderers of fathers and murderers of mothers, for manslayers, for whoremongers, for them that defile themselves with mankind, for men-stealers, for liars, for perjured persons, and if there be any other thing that is contrary to sound doctrine,” (1 Tim. i: 9, 10). He thus indicates that it is a restraint on unruly lusts that would otherwise burst all bonds.”

“11. To both may be applied the declaration of the Apostle in another place, that "The law was our schoolmaster to bring us unto Christ" (Gal. iii: 24); since there are two classes of persons, whom by its training it leads to Christ. Some (of whom we spoke in the first place), from excessive confidence in their own virtue or righteousness, are unfit to receive the grace of Christ, until they are completely humbled. This the law does by making them sensible of their misery, and so disposing them to long for what they previously imagined they did not want. Others have need of a bridle to restrain them from giving full scope to their passions, and thereby utterly losing all desire after righteousness. For where the Spirit of God rules not, the lusts sometimes so burst forth, as to threaten to drown the soul subjected to them in forgetfulness and contempt of God; and so they would, did not God interpose with this remedy. Those, therefore, whom he has destined to the inheritance of his kingdom, if he does not immediately regenerate, he, through the works of the law, preserves in fear, against the time of his visitation, not, indeed, that pure and chaste fear which his children ought to have, but a fear useful to the extent of instructing them in true piety according to their capacity. Of this we have so many proofs, that there is not the least need of an example. For all who have remained for some time in ignorance of God will confess, as the result of their own experience, that the law had the effect of keeping them in some degree in the fear and reverence of God, till, being regenerated by his Spirit, they began to love him from the heart.”

“12. The third use of the Law (being also the principal use, and more closely connected with its proper end) has respect to believers in whose hearts the Spirit of God already flourishes and reigns. For although the Law is written and engraven on their hearts by the finger of God, that is, although they are so influenced and actuated by the Spirit, that they desire to obey God, there are two ways in which they still profit in the Law. For it is the best instrument for enabling them daily to learn with greater truth and certainty what that will of the Lord is which they aspire to follow, and to confirm them in this knowledge; just as a servant who desires with all his soul to approve himself to his master, must still observe, and be careful to ascertain his master's dispositions, that he may comport himself in accommodation to them. Let none of us deem ourselves exempt from this necessity, for none have as yet attained to such a degree of wisdom, as that they may not, by the daily instruction of the Law, advance to a purer knowledge of the Divine will. Then, because we need not doctrine merely, but exhortation also, the servant of God will derive this further advantage from the Law: by frequently meditating upon it, he will be excited to obedience, and confirmed in it, and so drawn away from the slippery paths of sin. In this way must the saints press onward, since, however great the alacrity with which, under the Spirit, they hasten toward righteousness, they are retarded by the sluggishness of the flesh, and make less progress than they ought. The Law acts like a whip to the flesh, urging it on as men do a lazy sluggish ass. Even in the case of a spiritual man, inasmuch as he is still burdened with the weight of the flesh, the Law is a constant stimulus, pricking him forward when he would indulge in sloth. David had this use in view when he pronounced this high eulogium on the Law, “The law of the Lord is perfect, converting the soul: the testimony of the Lord is sure, making wise the simple. The statutes of the Lord are right, rejoicing the heart: the commandment of the Lord is pure, enlightening the eyes,” (Ps. xix: 7, 8). Again, “Thy word is a lamp unto my feet, and a light unto my path,” (Ps. cxix: 105). The whole psalm abounds in passages to the same effect. Such passages are not inconsistent with those of Paul, which show not the utility of the law to the regenerate, but what it is able of itself to bestow. The object of the Psalmist is to celebrate the advantages which the Lord, by means of his law, bestows on those whom he inwardly inspires with a love of obedience. And he adverts not to the mere precepts, but also to the promise annexed to them, which alone makes that sweet which in itself is bitter. For what is less attractive than the law, when, by its demands and threatening, it overawes the soul, and fills it with terror? David specially shows that in the law he saw the Mediator, without whom it gives no pleasure or delight.”

(Calvin, John. Institutes of the Christian Religion, Grand Rapids, MI: Wm. B. Eerdmans Publishing, 1998, p. 307-310.)