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 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. 19:7, 8). Again, “Thy word is a lamp unto my feet, and a light unto my path,” (Ps. 119:105). 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; Institutes; II.vii.12