(This post was amended on 2017-05-16 to add quote from Bente's Historical Introduction)
"Re Natural Law: if one minimizes the Law, also by denying natural law, one minimizes the Gospel."
- Therefore we conclude that a man is justified by faith without the deeds of the law. – Romans 3:28
- For what the law could not do, [Luther- "for what was impossible for the Law"...] in that it was weak through the flesh,... – Romans 8:3
The Lutheran Confessions speak on this also (as Pieper quotes C.D. II, 458) in the Apology (or Defense) of the Augsburg Confession, Article III, paragraphs 144-145:
 But works become conspicuous among men. Human reason naturally admires these, and because it sees only works, and does not understand or consider faith, it dreams accordingly that these works merit remission of sins and justify. This opinion of the Law inheres by nature in men's minds;  neither can it be expelled, unless when we are divinely taught. But the mind must be recalled from such carnal opinions to the Word of God. We see that the Gospel and the promise concerning Christ have been laid before us. When, therefore, the Law is preached, when works are enjoined, we should not spurn the promise concerning Christ. But the latter must first be apprehended, in order that we may be able to produce good works, and our works may please God, as Christ says, John 15:5: Without Me ye can do nothing.And Franz Pieper says it best when he wrote:
Only by hearing the Gospel is conversion brought about; no man can remain in the state of conversion unless he continues to use the Gospel.... men will be converted to God only when Law and Gospel are preached in the proper order and with the proper distinction. – (Christian Dogmatics, II, 459):
Who will have all men to be saved, and to come unto the knowledge of the truth. 1 Tim. 2:4
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2017-05-16 Addendum: See also F. Bente's Historical Introductions to the Lutheran Confessions (Triglotta p. 161) where he states in section XVII. The Antinomistic Controversy, #183:
“Wherever the Law is despised, disparaged, and corrupted, the Gospel, too, cannot be kept intact. Whenever the Law is assailed, even if this be done in the name of the Gospel, the latter is, in reality, hit harder than the former. The cocoon of antinomianism always bursts into antigospelism.”This is brought out in Eugene Klug's Getting into the Formula of Concord, pp. 47-48.