It was necessary for the direction of human life that, beyond natural and human law, there should be a divine law. There are four reasons for this need.
First, it is through law that man is directed to the acts which are proper to him in view of his ultimate end. If man were ordered only to an end which did not exceed his natural faculties, it would not be necessary for him to have any rational direction beyond natural law and that human law derived from it. Since, however, man is ordered to the end of eternal beatitude, which exceeds natural human faculties, it was necessary that he be directed to this end by divinely-given law in addition to natural and human law.
Second, due to the uncertainty of human judgment, particularly regarding contingent and particular things, there tend to be differing judgments regarding human acts, from which proceed diverse and contrary laws. Therefore, in order for man to be secure in the knowledge of what should be done and what avoided, it was necessary that his acts be directed by a divinely-given law which cannot err.
Third, man can make laws in those areas where he is competent to judge. His judgment does not extend to interior acts which lie hidden, however, but only to exterior acts which are apparent. Nevertheless, perfect virtue involves righteousness in both. Thus human law was unable to curb and direct internal acts sufficiently, and it was necessary that divine law supervene in this task.
Fourth, as Augustine says, human law cannot punish or prohibit all things that are evil, for in the process of removing evil it would also eliminate much that is good and impede the advancement of the common good, thus hurting society. Thus, in order to leave no evil unprohibited and unpunished, a divine law, through which all sins are punished, had to intervene.
These four causes are touched upon in the Psalm, where it is said (Ps. 18:8), “The law of the Lord is unspotted” (that is, permitting no foulness of sin), “converting souls” (because it directs not only exterior but also interior acts), “the testimony of the Lord is sure” (due to certainty of truth and rectitude), “giving wisdom to little ones” (insofar as it orders man to a supernatural and divine end).