"Man should be directed to his end by a law given by God"

"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 (Q. 5, A. 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."

~St. Thomas Aquinas: Summa theologiae, I-II, Q. 91, A. 4.