One of the textbook differences between Lutheran and Reformed theology is the Reformed assertion of the “third use” of the law, that is that the law serves to guide the conduct of believers. Thus, there is a way in which law and gospel co-exist in the believer’s life. Lutherans emphasize the contrast between law and gospel. Law comes “before” gospel in the sense that it prepares the sinner to receive the gospel. Gospel is deliverance from the law and the beginning of life in the Spirit. Part of the disagreement seems to me to derive from different descriptions of the content and function of the law. Randall Zachman gets at the problem:
Although both Luther and Calvin agree that the Old Testament contains attestations of God’s mercy as well as commandments and threats, it is nonetheless true that when Luther thinks of the law of Moses he thinks of the Ten Commandments in their theological use, whereas when Calvin thinks of the law of Moses he thinks of “the form of religion handed down by God through Moses” that sets forth God as Father to Israel in Christ under the double image of the tribe of Levi and the posterity of David.
This means that Calvin, in contrast to Luther, forces us to understand the Ten Commandments not as prior to, but as already contained within, the self-revelation of God the Father in Jesus Christ. This point will have direct implications on how Calvin understands the impact of the law in the narrow sense upon the conscience. On the one hand, it will mean that we cannot acknowledge that we are sinners who lack every good thing unless we at the same time know God as Father; on the other hand, it will mean that the principal use and proper purpose of the law will be in the lives of those who have already been adopted as children by the Father, and that this third use of the law will itself be given a christological meaning and shape just like the rest of the law of Moses. (The Assurance of Faith, 144)
Often the form of the question we ask determines the shape of the answer. An investigation into how a person might be assured of the favor of God will quickly (on Protestant principles) lead to a contrast between law and gospel. Likewise, an inquiry into the principles of Christian life seems to sputter or descend into subjectivism if “law” in some broad sense does not enter the picture. Perhaps Christian theology needs multiple perspectives on law and gospel in order to handle every theological problem. I find it likely, though, that one conception will end up becoming architectonic, while others wills be slotted into it to fill out the picture. So far, I’m more confident in the ability of the Reformed system to appropriate the Lutheran contribution than vice versa.