What relationship does the Christian have to God’s law – the law which is summarized in the Ten Commandments? A good place start is with Paul. He says that Christians are not under the law (Rom. 6.14, Gal. 5.18). According to Reformed theology, this means that we are not under the law as a covenant of works (WCF 19.6) . In other words, we aren’t justified by obeying the law. Another way we’re not under the law is in the national, theocratic sense. Deuteronomy is not our constitution like it was Israel’s constitution (cf. BCF 25, WCF 19.3-4).
One thing the law does is show us our sin (Rom. 3.20). The Heidelberg Catechism says that we come to know our misery through the law of God (Q/A 3; cf. CoD III/IV.5). So confessional Reformed/Presbyterian use the law in the liturgy. Part of worship, therefore, is to confess sins to God and repent of them as a church, publicly.
Another place the law shows up in Reformed theology is in the discussions on sanctification. The law of God is needed in the Christian life not just to show sin, but also to guide in grateful obedience. Though Psalm 119 talks about God’s word in general, it also specifically mentions loving God’s commands (119.47). The Psalmist also prays that God would lead him in obedience to the law (119.35). Jesus, John, James, and Paul describe love as the heartbeat of the law (Matt 22.37-40, Gal 5.14, James 2.8, and 1 Jn 4.11). Though the law doesn’t give us the power to obey it, it does show the Christian what is good, right, and pleasing to God. And so the law is in Reformed catechisms that even children learn. If we can call the law a map, we’d say the Spirit gives us the energy to go the direction the map tells us. The law tells us the good and right way, and the Spirit gives us the feet to walk the good and right way (Gal. 5.16, 25) to show our thankfulness and bring God glory.
Here’s how a few Reformed confessions (and one Lutheran one) explain this use of the law (a guide for gratitude or the third use of the law):
“[Good works are done] for the glory of God, to adorn our calling, to show gratitude to God, and for the profit of the neighbor” (2nd Helvetic Confession art. 16).
“By them [good works] believers manifest their thankfulness…and glorify God” (WCF 16.2; cf. WLC Q/A 32).
“We do good…so that in all our living we may show that we are thankful to God for all he has done for us and so that he may be praised through us” (HC Q/A 86).
“We need to remind [Christians] of how necessary it is that they exercise themselves in good works as a declaration of faith (Matt. 5.16) and gratitude to God (Heb. 13.15-16)” (Formula of Concord, Epitome, V).
These theologians are also worth listening to.
“Though the law not be the Christian’s savior, it is his guide. Though it not be a foedus, a covenant of life, yet it is a norma, a rule of life” (Thomas Watson).
“No longer capable of condemning us in God’s courtroom, the law directs our steps in the way of faith-filled gratitude” (Michael Horton).
“If we think of Christ as Paul here depicts him [in Gal. 4]…we shall understand why a Christian observes [God's] laws: for the peace of the world, out of gratitude toward God, and for a good example that others may be attracted to the gospel” (Martin Luther).
“True conversion cannot be without good works, and we in this way especially show our gratitude to God” (Zacharius Ursinus).
“In the first covenant [the covenant of works], man was bound to do this in order that he might live (to deserve life); but in this [covenant of grace] he is bound to do the same (not that he may live, but because he lives) to the possession of the life acquired by Christ and the testimony of a grateful mind” (Francis Turretin).
And the list goes on. The third use of the law – the law as a guide for Christian gratitude to God for his saving grace – is one of those important strands of Reformed theology. Take this strand out of Reformed theology and you end up with legalism, antinomianism, and a host of other tragic errors. Christians should know the law of God, most definitely. But they should also know the uses of it: to show them their sin, driving them to Christ, and to show them how to thank Christ in faith-filled obedience. For further study on this, read the last section of the Heidelberg Catechism or work your way through the relevant sections of G. Bethune’s commentary on the Heidelberg Catechsim.