When I was at seminary, we never studied law and gospel and their relation to each other systematically. We hardly touched on the subject in dogmatics, were given but a few journal articles on it to read in homiletics, and in our study of the Formula of Concord the explication of the relevant thesis was assigned to a student presentation, duration 50 minutes.
Whether this situation was a blip in the education of my particular class which came about accidentally through a lack of co-ordination on the part of the faculty, or whether it represented a more serious oversight I don't know, but it certainly represented a shortcoming in the education of one class of future Lutheran pastors and would-be 'seelsorgers' (healers of souls) in an area Luther regarded as the highest theological art.
Be that as it may, the following would have been a good place to start a study of the subject, both historically and dogmatically: 33 theses on law and gospel from the pen of the Reformer's friend, some-time mentor and indispensable assistant in the work of the Reformation, Philipp Melanchthon (the pic is of Lucas Cranach's 'Gesetz und Gnade' (Law & Grace) - a fascinating picture which, unfortunately, I can't display in a larger format here.)
Let us bring this whole discussion of law, gospel, and faith together under several theses:
1. The law is the doctrine that commands what is and what is not to be done.
2. The gospel is the promise of the grace of God.
3. The law demands impossible things such as the love of God and our neighbour.
4. Those who try to keep the law by their natural powers or free will simulate only the external works; they do not give expression to those attitudes which the law demands.
5. Therefore, they do not satisfy the law, but they are hypocrites, "whitewashed tombs," as Christ calls them in Matt 23:27. Gal 3:10 says: "For all who rely on the works of the law are under a curse.
6. Therefore, it is not the function of the law to justify.
7. But the proper function of the law is to reveal sin and especially to confound the conscience. Rom 3:20: "Through the law comes knowledge of sin."
8. To a conscience acknowledging sin and confounded by the law, the gospel reveals Christ.
9. Thus John reveals Christ at the very time he preaches repentance: "Behold the Lamb of God, who takes away the sin of the world" (John 1:29).
10. The faith by which we belive the gospel showing us Christ and by which Christ is received as the one who has placated the Father and through whom grace is given, this faith is our righteousness. John 1:12: "But to all who received him, who believed in his name, he gave power to become Children of God."
11. If it is actually faith alone that justifies, there is clearly no regard for our merits or our works , but only for the merits of Christ.
12. This faith calms and gladdens the heart. Rom 5:1: "Therefore, since we are justified by faith, we have peace."
13. The result of faith is that for such a great blessing, the forgiveness of sins because of Christ, we love God in return. Therefore, love for God is a fruit of faith.
14. This same faith causes us to be ashamed of having offended such a kind and generous father.
15. Therefore, it cause us to abhor our flesh with its evil desires.
16. Human reason neither fears God nor believes him, but is utterly ignorant of him and despises him. We know this from Ps. 14:1: "The fool says in his heart, 'There is no God.'" Luke 16:31 "If they do not hear Moses and the prophets, neither will they be convinced if someone should rise from the dead." Here Christ points out that the human heartdoes not believe the word of God. This madness of the human heart is what Solomon railed at in the whole book of Ecclesiastes as can be seen from ch. 8:11: "Because sentence agains an evil deed is not executed speedily, the heart of the sons of men is fully set to do evil."
17. Because the human heart is utterly ignorant of God, it turns aside to its own counsels and desires, and sets itself up in the place of God.
18. When God confounds the human heart through the law with a sense of sin, it does not yet know God, that is, it does not know his goodness and therefore hates him as if he were a tormentor.
19. When God comforts and consoles the human heart through the gospel by showing it Christ, then finally it knows God, for it recognizes both his power and his goodness. This is what Jer 9:24 means: "But let him who glories glory in this, that...he knows me."
20. The heart of him who has believed the gospel and come to know the goodness of God is now fortified so that it trusts in God and fears him and consequently abhors the thoughts of the human heart.
21. Peter said very fittingly in Acts 15:9 that hearts are cleansed by faith.
22. Mercy is revealed through the promises.
23. Sometimes material things are promised, and at other times spiritual.
24. In the law, material things such as the Land of Canaan, the Kingdom, etc. are promised.
25. The gospel is the promise of grace or the forgiveness of sins through Christ.
26. All material promises are dependent on the promise of Christ.
27. For the first promise was a promise of grace or Christ. It is found in Gen 3:15: "He shall bruise your head." This means that the seed of Eve will crush the kingdom of the serpent plotting agains our heel, that is Christ will crush sin and death.
28. This was renewed in the promise made to Abraham: "By your decendants shall all the nations of the earth be blessed" (Gen. 22:18).
29. Therefore, since Christ was to be born of the descendants of Abraham, the promises added to the law about the possession of the earth, etc. were obscure promises of the Christ who was to come. For those material things were promised to the people until the promised seed should be born, lest they perish and in order that in the meantime God might indicate his mercy by material things and might thereby exercise the faith of his people.
30. By Christ's birth the promises to mankind were consumated, and the forgiveness of sins, for which Christ had to be born, was openly made known.
31. The promises of the Old Testament are signs of the Christ to come and also of the promise of grace to be broadcast at some future time. The gospel, the very promise of grace, has already been made known.
32. Just as that man does not know God who knows only that he exists but does not know either his power or his mercy, so also that man does not believe who believes only that God exists but does not believe both in his power and his mercy.
33. He really believes, therefore, who, looking beyond the threats, believes the gospel also, who fixes his face on the mercy of God or on Christ, the pledge of divine mercy.
So much on faith; we shall add certain things on love a little later after we have dealt with the difference between the law and gospel.
From Melanchthon's Loci communes rerum theologicarum (Common Places of Theology), generally recognised as the first Reformation dogmatics text. It is a mark of Melanchthon's brilliance that he published the first edition of this work just two years after being awarded his bachelor's degree in theology.