Objection 1. It would seem that there was no need for a Divine law. Because, as stated above (Article 2), the natural law is a participation in us of the eternal law. But the eternal law is a Divine law, as stated above (Article 1). Therefore there was no need for a Divine law in addition to the natural law, and human laws derived therefrom.
Objection 2. Further, it is written (Sirach 15:14) that "God left man in the hand of his own counsel." Now counsel is an act of reason, as stated above (Question 14, Article 1). Therefore man was left to the direction of his reason. But a dictate of human reason is a human law as stated above (Article 3). Therefore there is no need for man to be governed also by a Divine law.
Objection 3. Further, human nature is more self-sufficing than irrational creatures. But irrational creatures have no Divine law besides the natural inclination impressed on them. Much less, therefore, should the rational creature have a Divine law in addition to the natural law.
On the contrary, David prayed God to set His law before him, saying (Psalm 118:33): "Set before me for a law the way of Thy justifications, O Lord."
I answer that, Besides the natural and the human law it was necessary for the directing of human conduct to have a Divine law. And this for four reasons. First, because it is by law that man is directed how to perform his proper acts in view of his last end. And indeed if man were ordained to no other end than that which is proportionate to his natural faculty, there would be no need for man to have any further direction of the part of his reason, besides the natural law and human law which is derived from it. But since man is ordained to an end of eternal happiness which is inproportionate to man's natural faculty, as stated above (Question 5, Article 5), therefore it was necessary that, besides the natural and the human law, man should be directed to his end by a law given by God.
Secondly, because, on account of the uncertainty of human judgment, especially on contingent and particular matters, different people form different judgments on human acts; whence also different and contrary laws result. In order, therefore, that man may know without any doubt what he ought to do and what he ought to avoid, it was necessary for man to be directed in his proper acts by a law given by God, for it is certain that such a law cannot err.
Thirdly, because man can make laws in those matters of which he is competent to judge. But man is not competent to judge of interior movements, that are hidden, but only of exterior acts which appear: and yet for the perfection of virtue it is necessary for man to conduct himself aright in both kinds of acts. Consequently human law could not sufficiently curb and direct interior acts; and it was necessary for this purpose that a Divine law should supervene.
Fourthly, because, as Augustine says (De Lib. Arb. i, 5,6), human law cannot punish or forbid all evil deeds: since while aiming at doing away with all evils, it would do away with many good things, and would hinder the advance of the common good, which is necessary for human intercourse. In order, therefore, that no evil might remain unforbidden and unpunished, it was necessary for the Divine law to supervene, whereby all sins are forbidden.
And these four causes are touched upon in Psalm 118:8, where it is said: "The law of the Lord is unspotted," i.e. allowing no foulness of sin; "converting souls," because it directs not only exterior, but also interior acts; "the testimony of the Lord is faithful," because of the certainty of what is true and right; "giving wisdom to little ones," by directing man to an end supernatural and Divine.
Reply to Objection 1. By the natural law the eternal law is participated proportionately to the capacity of human nature. But to his supernatural end man needs to be directed in a yet higher way. Hence the additional law given by God, whereby man shares more perfectly in the eternal law.
Reply to Objection 2. Counsel is a kind of inquiry: hence it must proceed from some principles. Nor is it enough for it to proceed from principles imparted by nature, which are the precepts of the natural law, for the reasons given above: but there is need for certain additional principles, namely, the precepts of the Divine law.
Reply to Objection 3. Irrational creatures are not ordained to an end higher than that which is proportionate to their natural powers: consequently the comparison fails.
Source: Summa Theologica (New Advent)