Section I: The Name “Moral Law,” § 13.
SECTION I: THE NAME “MORAL LAW”
The sum of the moral Law [is] in the Decalogue.
§ 13. The sum of the moral Law is in the Decalogue, which God taught to the Israelites on Mount Sinai amid the blaring trumpets and other marvelous signs (Exod. 19:20). The word “Decalogue” is taken from the passages of Scripture where the moral Law, revealed and repeated on Mount Sinai, is called 
(asereth haddebarim, “the decade of words” or “the Ten Words,” Exod. 34:28; Deut. 4:13; 10:4). Philo has a nice play on words (De decalogo, p. 475): “The men who first gave names to things were wise men. They seemed to have named this rightly ‘a decade’ [δεκάδα], like a box [δεχάδα] that holds and includes all kinds of numbers and computations and arithmetic proportions as well as harmonies and symphonies,” etc. Therefore just as the number ten is the most perfect and all-inclusive, so also the moral Law, which God in His wisdom put into the Ten Commandments, is most perfect. Bede (on Exodus 20): “No number grows larger except up to ten. In this way God established the fullness of the commandments in the fullness of numbers.” Spinaeus writes (De providentia): “Philo noted well that in the Ten Commandments far more is comprehended and taught than all that is contained in the countless volumes about this subject written by philosophers, legislators, and lawyers. In those tomes there is not a word about preserving justice, or about the right way to establish republics, for they contain nothing richer and more learned than those Ten Commandments.” Therefore what Cicero (De orat., bk. 1) says about the Laws of the Twelve Tables1 can be said far more correctly about the moral Law as it is set forth in the ten words of the Decalogue: “The one little booklet of the Twelve Tables surpasses all the libraries of the philosophers in both the weight of its authority and the richness of its utility.” Biel (Sent., 3, distinct. 37, q. 11, art. 6): “All other precepts, divine as well as human, can be traced back to the Decalogue as their foundation, conclusion, or as rationally harmonious with them.”
1 The Twelve Tables, now surviving only in fragments, were the ancient basis for Roman law and the Roman republic.