It is law that establishes the standards which men must obey if they would respect their obligations and not transgress their privileges. Man is obliged to other men and he is obliged to God. Law determines the extent and limits of his obligation and tells him what he must do.
Law is simply the determination of individual obligation as laid down by a legitimate authority.
Most people are humble enough to recognize that the good of society at large must be preferred to the individual good. If this were not recognized, we should have anarchy, in which individual rights would be destroyed. Similarly, most people recognize that God has claims upon man. The recognition of those claims we call God's laws. Law, which reason tells us is necessary in the affairs of men, thus has a solid New Testament foundation as well in signifying man's duties towards God.