Here’s a nice section from Mueller’s Dogmatics I found digging around for more references to the opinio legis. Enjoy.
4. THE TWO SOURCES (PRINCIPIA COGNOSCENDI) OF THE EXISTING RELIGIONS.
As we have seen, there are but two essentially different religions, the religion of faith, or of the Gospel, and the religion of works, or of the Law. So also there are but two actual sources (principia cognoscendi, principles of knowledge) from which these two divergent religions are taken. The religion of works is of human origin; it is a man-made religion, having its source and origin in the human heart, in which God has inscribed His divine Law, so that also the heathen, who have not the Word of God as set forth in Holy Scripture, Rom. 2:14, “know the judgment of God” (Grk: dixaicona, the norm of right, Rechtssatzung) , Rom. 1:32, and “show the work of the Law written in their hearts,” Rom. 2:15.
On the basis of the divine Law, inscribed in the human heart, conscience accuses and condemns man whenever he does wrong, and so he is burdened with the consciousness of guilt, “they are without excuse” Rom. 1, 20, “their conscience also bearing witness and their thoughts the mean while accusing or else excusing one another,” Rom. 2:15.
Thus condemned by his conscience, man seeks to reconcile the Deity by “good works,” such as worship, sacrifices, etc. The Apology rightly says: “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 (haec opinio legis) inheres by nature in men’s minds; neither can it be expelled, unless when we are divinely taught. But the mind must be recalled (revocanda mens est) from such carnal opinions of the Word of God.” (Art. III.144.)
The “opinion of the Law” of which the Apology here speaks, namely, the erroneous view that works merit remission of sins and justify the sinner, St. Paul calls “the religion of the flesh.” So he writes to the Galatians, who sought justification on the ground of their merits: “Are ye so foolish? Having begun in the Spirit, are ye now made perfect by the flesh?” (Gal. 3:3). Luther correctly explains this passage as follows: “Here flesh is nothing else than the righteousness, the wisdom, of the flesh and the thoughts of reason, which endeavors to be justified by the Law.” (St. L. Ed., IX, 288 ff.) That this is indeed the meaning of the word flesh in this passage the context clearly proves. The passage thus teaches the truth that every religion which seeks to acquire divine grace and remission of sins through human endeavors is not of God, but of man. Its source is the perverted, unregenerate heart.
The religion of the Gospel, or of faith, on the contrary, is not of man, but of God, who has revealed it by His inspired prophets and apostles in Holy Scripture. 1 Cor. 2:6-10: “We speak wisdom among them that are perfect; yet not the wisdom of this world; . . . but we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory; which none of the princes of this world knew. … But as it is written, Eye hath not seen nor ear heard, neither have entered into the heart of man the things which God hath prepared for them that love Him. But God hath revealed them unto us by His Spirit,” etc.
(Find the entire text online here: https://archive.org/details/Mueller/page/n3)
