(_Summa Theologica_, I-II, qq. 98-108.)
328. Meaning.—The Positive Divine Law is the law added by God to the Natural Law, in order to direct the actions of man to his supernatural End, to assist him to a better observance of the Natural Law, and to perfect that which is wanting in human law.
(a) The Last End of man is not natural, but supernatural (see 20), and hence it was necessary that, in addition to the precepts which guide man towards his natural beatitude, there should be added precepts that will guide him towards his supernatural beatitude: “The Law of the Lord gives wisdom to little ones” (Ps. xviii. 8).
(b) The light of natural reason was sufficient to instruct man in the Natural Law, but through sin that light had become obscured, with the result that evil customs set in, and very many were at a loss how to apply the Natural Law, or applied it wrongly. Hence, it was most suitable that the Natural Law should be summed up in brief commandments and given externally by the authority of God. This was done through the Decalogue, which is a part of the Positive Divine Law of both the Mosaic and the Christian dispensations: “The testimony of the Lord is faithful” (Ps. xviii. 8).
(c) Human laws are the product of fallible human judgment; they can direct only such acts as are external, and they are unable to forbid or punish many evil deeds. Hence, it was necessary that there should be positive divine laws to supply for what is wanting in human law: “The law of the Lord is unspotted, converting souls” (Ps. xviii. 8).
329. The Positive Divine Law differs from the Natural Law as to subject-matter, permanence, and manner of promulgation.
(a) The precepts of the Natural Law are necessary, since they follow as necessary consequences from the nature of man, the precepts of the Positive Law of God, excluding those that are external promulgations of the Natural Law, are not necessary, since they follow from the free decree of God raising man to that which is above his nature.
(b) The precepts of the Natural Law are unchangeable, since the nature of man always remains the same. Of the precepts of the Positive Law of God some were changed, because given only for a time (such as the ceremonial laws of Judaism); others, absolutely speaking, could be changed, because not necessarily connected with the end God has in view (e.g., the laws concerning Sacraments).
(c) The precepts of both kinds of law are immediately from God; but the Natural Law is promulgated only in a general way, through the light of reason given to man along with his nature, while the Positive Law of God is proclaimed by special commands (e.g., “thou shalt not steal”).
330. The Positive Divine Law contains two kinds of precepts, viz., natural and supernatural commandments. (a) The natural precepts were given in order to recall to the minds of men the laws knowable through reason which had become obscured through passion, custom or example. The Commandments given to Moses on the tablets of stone renewed the natural precepts which God had written through reason on the hearts of men. (b) The supernatural precepts were given in order to point out to men the duties their supernatural destiny imposed. Example: The precepts of faith, hope, charity.