“Mores” (the plural of mos), are the group of standards or norms that determine right and wrong. It was first used by Cicero (De Fato, II.i) to translate the Greek ethikos. “Moral” means “according to the mores.”
When used of human societies this is a subjective groups of norms, pertinent to the group or culture from which they spring. When used by Christians it refers to the character of God as the norm or standard which determines what is good and right.
Moral law then, is law that springs from the unchanging nature of God as the norm by which good or evil is determined. That which accords with the character of God is good and right. That which is out of sync with the character of God is evil or wrong.
Liberty is not freedom from rules but the right to live according to the law and free from tyranny, which rides roughshod over the law.
Moral law provides the structure of freedom. Liberty is living freely within that structure.