In contrast to the myth of "natural law" we have Revealed Law, the Divine Revelation from our Creator to us. This is absolutely necessary, for 2 reasons:
1. Sin has blinded man. His reason is not what dictates his actions. Man's volition is in bondage to sin. This bondage is voluntary, but that by no means makes it less real. Hence, man will "reason" his way to sin and justifying his sin.
Testimony from God:
Jeremiah 17:9 KJV
The heart is deceitful above all things, and desperately wicked: who can know it?
Since the heart is deceitful and desperately wicked, man requires an EXTERNAL source of Law. Left to his own devices, he will promote wickedness and enshrine iniquity into law. Divine Revelation is required for man to understand moral obligation, right and wrong.
2. Law is either moral, or physical. Physical law respects automatic processes, as the law of gravity, the laws governing sensory perception, the laws of motion, and chemistry, etc. Moral law respects the conduct of moral agents, it governs the choices men make. It is the law of right and wrong, what men should or should not do. Moral law is voluntary, a man may choose to do what is wrong.
Man, being a moral agent, is a subject of moral law. Since moral law governs choice and the will, it must originate in a moral agent. Physical processes do not give rise to moral law anymore than inert matter spontaneously generates life. Thus, moral law must come from a Lawgiver. There is only one universal lawgiver, God. Therefore, universal moral law must come from Him. The lawgiver must communicate His law to His subjects. That communication is Divine Revelation, it is the Holy Bible or WORD OF GOD. Therefore, moral law is found in the Word of God, not in the corrupted fallible reasonings of the subjects of moral law.
And therefore, appeals to "natural law" are unneeded and unuseful, and a distraction from finding genuine moral law. Instead, the Word must be resorted to as the sure communication from the Sovereign to His subjects.