Jon Payne has now written his second post on sanctification and the Heidelberg Catechism (HC) at Reformation 21. In his first post (to which I responded here), he discussed the variety of motivations for Christian obedience, which extend beyond mere gratitude for our justification (as important as that is). Now, he takes up the subjects of the “third use” of the law and the role of effort in sanctification.
For readers unfamiliar with this threefold distinction in Reformed theology, God’s moral law can be used as: 1) a mirror, exposing our sin and revealing our need of Christ; 2) a curb, restraining our sin and enabling society to function; and 3) a guide, directing believers in the path of obedience.  Rather than fostering a sense of works-righteousness (a common misconception), the third use of the law emphasizes the abiding beauty and authority of God’s law for believers. To be a Christian is not to be free from God’s law, although it is to be free from its curse.
Payne discusses the “law-gospel-law” schema as it is used in Reformed evangelism and preaching. According to this traditional understanding, the law should be proclaimed first to tear us down, so that the gospel can then build us up. And then, having a clear view of ourselves as sinners and of Christ as our Savior, we can return to the law, not as a means of justification, but rather as a rule of gratitude. This homiletic strategy is valid as far as it goes, but I do wonder if it is somewhat simplistic. As John Frame points out in his article “Law and Gospel,” Scripture often gives the gospel a logical and temporal priority over law. For example, God gave Israel the law at Sinai after he had redeemed them from slavery in Egypt (Ex. 19-20). And in John 4, Jesus offers living water to the Samaritan woman at the well, and then exposes her sins. Moreover, the gospel is not merely a message of comfort and forgiveness. It is a proclamation of Christ’s victory over his enemies, and as such it is a warning against those who would rebel against his kingdom. This is why Scripture sometimes issues threats against those who do not “obey the gospel” (2 Th. 1:8; 1 Pet. 4:17). 
In a similar vein, Dutch theologian G.C. Berkouwer writes that the gospel itself can have an “elenchtic” (convicting) use, as the disclosure of God’s self-sacrificial love in Christ breaks the selfish hearts of humans (Berkower, Studies in Dogmatics: Sin [Eerdmans, 1971]). When is the last time you heard a sermon preached on the terror of the gospel (or the comfort of the law, for that matter)?
As usual, the Puritans can be of help here. They distinguished between the law “taken strictly” and the law “taken largely,” as well as between the gospel “taken strictly” and the gospel “taken largely.” Taken strictly, the law is a bare requirement for perfection, which sinful humans are unable to keep. In this sense, it can do nothing but convict us for our imperfection. But taken largely, the law is received in the context of God’s redeeming and enabling grace, whereby God accepts sincere obedience as though it were perfect obedience. Likewise, the gospel taken strictly is an unconditional message of comfort and forgiveness, while the gospel taken largely includes the call to repentance and a warning to all who refuse to submit to the lordship of Christ (Joel Beeke and Mark Jones, A Puritan Theology: Doctrine for Life [Reformation Heritage, 2012]). Keeping in mind these differences in terminology might go a long way in preventing Reformed theologians today from talking past one another.
These caveats aside, Payne has written a great piece here, and it deserves our attention. He rightly cautions against an incipient “soft antinomianism” in the Reformed world today, and encourages us to incorporate all that Scripture has to say about Christian sanctification.
1. It is worth noting that, in the Lutheran tradition, the first two uses of the law are inverted. The Lutheran Formula of Concord describes the first use of the law as a curb, and the second use as a mirror. It is not uncommon to hear Calvinists slipping into the Lutheran taxonomy, but Calvin himself gives the order listed above in his Institutes of the Christian Religion 2.7.
2. It is sometimes said that gospel obedience is categorically different from law obedience. Indeed, Samuel Bolton, one of the Westminster divines, wrote of the distinction between “evangelical obedience” and “legal obedience” (see post here). Bolton describes evangelical obedience as a godly pursuit of obedience to the moral law, stemming from a genuine knowledge of oneself as a sinner saved by grace alone. By contrast, he describes legal obedience not as obedience to the law per se, but rather as a vain effort on the part of the unregenerate to establish their own righteousness by slavish law-keeping. Therefore, readers should not reduce “evangelical obedience” to mere “faith in the gospel,” as if it were distinct from obedience to God’s law.