Shorter Catechism Q. 40. What did God at first reveal to man for the rule of his obedience? A. The rule which God at first revealed to man for his obedience was the moral law.
Commentary: A law is a command that directs us, that is, it tells us what to do. It also binds us in that we are obligated to keep it. A law is not a suggestion or a good piece of advice. It is a requirement. Failure to keep it, therefore, is blameworthy and produces guilt.
The law, which God first revealed to man for his obedience, was the moral law. It is called moral to distinguish it from the ceremonial (religious/redemptive) laws and judicial or civil laws that were given by God through Moses to Israel at Mt. Sinai.
Moral law may be divided into natural moral law and positive moral law. Natural moral laws are those which are imbedded in creation, written on the human heart, and are a reflection or expression of the holy nature of God. They are universal, perpetual, perfect (complete), and unchangeable. Examples include love for God and worship of God. Positive moral laws are those which are not known naturally but must be revealed directly by God. They are subject to change or abrogation. The day of worship is an example of a positive moral law.
Westminster Larger Catechism 93 defines the moral law thus: “The moral law is the declaration of the will of God to mankind, directing and binding every one to personal, perfect, and perpetual conformity and obedience thereunto, in the frame and disposition of the whole man, soul and body, and in performance of all those duties of holiness and righteousness which he oweth to God and man: promising life upon the fulfilling, and threatening death upon the breach of it.”