But to make the whole matter clearer, let us survey briefly the function and use of what is called the “moral law.” Now, so far as I understand it, it consists of three parts.

The First Use of the Moral Law: Convict

The first part is this: while it shows God’s righteousness, that is, the righteousness alone acceptable to God, it warns, informs, convicts, and lastly condemns, every man of his own unrighteousness. . .

The law is like a mirror. In it we contemplate our weakness, then the iniquity arising from this, and finally the curse coming from both—just as a mirror shows us the spots on our face. . . 

The apostle’s statement is relevant here: “Through the law comes knowledge of sin” [Rom. 3:20]. There he notes only its first function, which sinners as yet unregenerate experience. Related to this are these statements: “Law slipped in, to increase the trespass” [Rom. 5:20], and thus it is “the dispensation of death” [2 Cor. 3:7] that “brings wrath” [Rom. 4:15], and slays. There is no doubt that the more clearly the conscience is struck with awareness of its sin, the more the iniquity grows. For stubborn disobedience against the Lawgiver is then added to transgression. It remains, then, to the law to arm God’s wrath for the sinner’s downfall, for of itself the law can only accuse, condemn, and destroy. As Augustine writes: “If the Spirit of grace is absent, the law is present only to accuse and kill us.”

(Note: Calvin emphasizes that the first use of the law, which continues to function throught the Christian's life (see below on the third use), is not meant to drive the children of God to despair and discouragement. Rather the "sting" of the law is intended to drive the children of God to flee to God's mercy in Christ. Calvin writes:

"The wickedness and condemnation of us all are sealed by the testimony of the law. Yet this is not done to cause us to fall down in despair or, completely discouraged, to rush headlong over the brink—provided we duly profit by the testimony of the law. It is true that in this way the wicked are terrified, but because of their obstinacy of heart. For the children of God the knowledge of the law should have another purpose. The apostle testifies that we are indeed condemned by the judgment of the law, “so that every mouth may be stopped, and the whole world may be held accountable to God” [Rom. 3:19]. He teaches the same idea in yet another place: “For God has shut up all men in unbelief,” not that he may destroy all or suffer all to perish, but “that he may have mercy upon all” [Rom. 11:32]. This means that, ^adismissing the stupid opinion of their own strength, they come to realize that they stand and are upheld by God’s hand alone; that, naked and empty-handed, they flee to his mercy, repose entirely in it, hide deep within it, and seize upon it alone for righteousness and merit. For God’s mercy is revealed in Christ to all who seek and wait upon it with true faith. ^bIn the precepts of the law, God is but the rewarder of perfect righteousness, which all of us lack, and conversely, the severe judge of evil deeds. But in Christ his face shines, full of grace and gentleness, even upon us poor and unworthy sinners."

Quoting Augustine, Calvin writes, "The usefulness of the law lies in convicting man of his infirmity and moving him to call upon the remedy of grace which is in Christ.”

The Second Use of the Moral Law: Restrain

The second function of the law is this: at least by fear of punishment to restrain certain men who are untouched by any care for what is just and right unless compelled by hearing the dire threats in the law.

But they are restrained, not because their inner mind is stirred or affected, but because, being bridled, so to speak, they keep their hands from outward activity, and hold inside the depravity that otherwise they would wantonly have indulged. Consequently, they are neither better nor more righteous before God. Hindered by fright or shame, they dare neither execute what they have conceived in their minds, nor openly breathe forth the rage of their lust. Still, they do not have hearts disposed to fear and obedience toward God.

The Third Use of the Moral Law: Guide

The third and principal use, which pertains more closely to the proper purpose of the law, finds its place among believers in whose hearts the Spirit of God already lives and reigns. For even though they have the law written and engraved upon their hearts by the finger of God [Jer. 31:33; Heb. 10:16], that is, have been so moved and quickened through the directing of the Spirit that they long to obey God, they still profit by the law in two ways.

1. Here is the best instrument for them to learn more thoroughly each day the nature of the Lord’s will to which they aspire, and to confirm them in the understanding of it.

2. Again, because we need not only teaching but also exhortation, the servant of God will also avail himself of this benefit of the law: by frequent meditation upon it to be aroused to obedience, be strengthened in it, and be drawn back from the slippery path of transgression. . .

(Note: In discussing the third use of the law, Calvin observes that the first use of the law remains even in the third use for Christians (see also Heidelberg Catechism, Q. 114-115).

Even for a spiritual man not yet free of the weight of the flesh the law remains a constant sting that will not let him stand still. Doubtless David was referring to this use when he sang the praises of the law: “The law of the Lord is spotless, converting souls; … the righteous acts of the Lord are right, rejoicing hearts; the precept of the Lord is clear, enlightening the eyes,” etc. [Ps. 18:8–9, Vg.; 19:7–8, EV]. Likewise: “Thy word is a lamp to my feet and a light to my path” [Ps. 119:105], and innumerable other sayings in the same psalm [e.g., Ps. 119:5]. These do not contradict Paul’s statements, which show not what use the law serves for the regenerate, but what it can of itself confer upon man. But here the prophet proclaims the great usefulness of the law: the Lord instructs by their reading of it those whom he inwardly instills with a readiness to obey. He lays hold not only of the precepts, but the accompanying promise of grace, which alone sweetens what is bitter. For what would be less lovable than the law if, with importuning and threatening alone, it troubled souls through fear, and distressed them through fright?)

(Note: Lastly, Calvin points out that the third use of the law does not function in the believer's life like a "rigorous enforcement officer." Refuting an overrealized eschatology (i.e., thinking you have now more blessings from the future than you actually do), Calvin writes,

"We ought not to be frightened away from the law or to shun its instruction merely because it requires a much stricter moral purity than we shall reach while we bear about with us the prison house of our body. For the law is not now acting toward us as a rigorous enforcement officer who is not satisfied unless the requirements are met. But in this perfection to which it exhorts us, the law points out the goal toward which throughout life we are to strive. In this the law is no less profitable than consistent with our duty. If we fail not in this struggle, it is well. Indeed, this whole life is a race [cf. 1 Cor. 9:24–26]; when its course has been run, the Lord will grant us to attain that goal to which our efforts now press forward from afar."

*The preceding quotes are taken from John T. McNeill, Calvin: Institutes of the Christian Religion, I:2.7.6-13.