Through the Fall the absolute knowledge of the divine will which God at creation had planted into the human soul was greatly weakened or obscured. For this reason man after the Fall no longer knows the divine will, or Law, with certainty, though his conscience (συνείδησις, conscientia) in a measure still functions, Rom. 2, 14. 15. Moreover, after the Fall, conscience may err (conscientia erronea), so that man often regards as forbidden what God allows (eating of certain foods at certain times, drinking of spirituous liquors, etc.), or, vice versa, regards as allowed what He has forbidden (worshiping idols, trusting in one's works for salvation). So also conscience may entertain doubts (conscientia dubio.) with regard to the propriety of certain acts, or it may suggest no more than a mere probability (conscientia probabilis) of right or wrong, so that man remains uncertain with regard to the course which he must follow. Conscience, after the Fall, is therefore no longer a safe standard of what God wills or forbids. The only inerrant norm by which God's immutable will may be known with certainty is Holy Scripture, which contains a complete revelation of the divine Law, Matt. 5, 18. 19; Gal. 3, 23. 24, though properly this was given to men for the sake of the Gospel, Rom. 3, 19-22.
From Holy Scripture we know with certainty which laws were meant to be temporary and which, on the other hand, all men at all times must obey, Col. 2, 16. 17; Gal. 5, 1. 2. The immutable will of God is the Moral Law, which binds all men and obligates them to obedience, Matt. 22,37-40; Rom. 13,8-10. While the Moral Law is summarily comprehended in the Decalog, the Ten Commandments, in the form in which they were given to the Jews, Ex. 20, 1-17, must not be identified with the Moral Law, since they contain ceremonial features, Ex. 20, 8-11; Deut. 5, 12-15. Only in its New Testament version may the Decalog be identified with the Moral Law, or the immutable will of God, Rom. 13, 8-10; J as. 2, 8; 1 Tim. 1, 5. (Cp. Luther, St. L., XX, 146 ff.)
It is self-evident that commandments given to individual be lievers (mandata specialia), Gen. 22, must not be interpreted as applying to men in general. That the Mosaic laws regarding the prohibited degrees of consanguinity and affinity, Lev. 18, pertained not only to the Jews, but to men in general is indicated by the text itself, Lev. 18, 24-30, though the levirate command was temporary, obligating only the children of Israel (Deut. 25, 5-10; cp. v. 10: "His name shall be called in Israel," etc.)