Reformed theologians commonly speak of three (3) uses of the law of God. In his Systematic Theology, Louis Berkhof (1873-1957) explains the distinctions between the various uses (what he calls the “threefold use”) in the following way:
Use #1 – the Civil Use: “The law serves the purpose of restraining sin and promoting righteousness. Considered from this point of view, the law presupposes sin and is necessary on account of sin. It serves the purpose of God’s common grace in the world at large” (p.614).
So in this way God’s law is useful for the benefit of society generally (both believer and unbeliever alike). Sin and wickedness have detrimental effects on any community or society. Righteousness, on the other hand, is beneficial to any community or society. As Proverbs 14:34 says, “Righteousness exalts a nation, but sin is a reproach to any people” (ESV). So the law of God (in order for the first use of the law to actually be of use in this way) must then somehow be published or made known to society in general. The less the law is made known, the less it will be of use to restrain sin (or to promote righteousness) in society. While such a use is certainly limited to common (not saving) grace, as Berkhof points out above, such common grace is a good thing. It should not be looked down upon or neglected.
Use #2 – the Pedagogical Use: “In this capacity the law serves the purpose of bringing man under conviction of sin, and of making him conscious of his inability to meet the demands of the law. In that way the law becomes a tutor to lead him unto Christ, and thus becomes subservient to God’s gracious purpose of redemption” (ibid).
The Westminster Larger Catechism Q.96 speaks of this use:
“What particular use is there of the moral law to unregenerate men? A. The moral law is of use to unregenerate men, to awaken their consciences to flee from wrath to come, and to drive them to Christ; or, upon their continuance in the estate and way of sin, to leave them inexcusable, and under the curse thereof.”
In this use the law of God acts as a mirror of sorts, showing the unbeliever his sin, and driving him to look to Christ by faith for salvation. The law shows us our desperate need for the Savior.
Use #3 – the Normative Use: “This is the so-called . . . third use of the law. The law is a rule of life for believers, reminding them of their duties and leading them in the way of life and salvation. This third use of the law is denied by the Antinomians” (p.615).
Simply put, after the law of God drives us to faith in Christ for salvation (2nd use of the law), it then becomes our rule of life (3rd use), showing us how we should live in light of our salvation in Christ. The Westminster Larger Catechism Q.97 speaks of this use:
“What special use is there of the moral law to the regenerate? A. Although they that are regenerate, and believe in Christ, be delivered from the moral law as a covenant of works, so as thereby they are neither justified nor condemned; yet besides the general uses thereof common to them with all men, it is of special use, to show them how much they are bound to Christ for his fulfilling it, and enduring the curse thereof in their stead, and for their good, and thereby to provoke them to more thankfulness, and to express the same in their greater care to conform themselves thereunto as the rule of their obedience.”
So to the regenerate (i.e. believers in Christ), the law is still of great usefulness, to show us our debt to Christ for our salvation (both in His active and passive obedience), to “provoke” us to thankfulness to Him for our salvation, and to express that thankfulness in striving to conform ourselves unto His law in our daily lives. In a sense, the believer now has even more reason to obey God’s law because of his salvation!