No Christian doubts that the contents of the law are good, the commandment holy, just and good. Further, when God acted, He more or less displayed His character, what He is; and, as a general principle, there must be goodness and loving-kindness, because these above all characterize Him. Even judgment will put an end to evil and be deliverance from evil, though in itself purely righteousness. The Cross gives the whole truth fully.
All this however is not the question; but what as such is law? It is requirement from man, not the revelation of God. In its dispensation, a God who had delivered may have uttered it; but it is requirement from man of what he ought to do, or prohibition of what he ought not to do. In the Decalogue (save one commandment, or at most two, 4 and 5) it is the latter; that is, it supposes and, in eight or nine tenths of it, speaks of sin.
Obligation does not rest on law, but on relationships: law maintains these; and so does Christ, but on a different principle God has formed men in certain relations to Himself and to one another. A good nature (and man was so formed) would naturally walk in them, because it was a conscious relationship by creation itself, and the communications connected with it. I add this because it is always true with God, another being was needed and existed.
Law takes up these relationships, with some other things consequent as facts on the Fall. Hence the Decalogue is not arbitrary, but God's maintenance of the relationships in which He had placed man, in the circumstances in which he now found himself. It is a perfect rule for the child of Adam as he is, as a summary of moral duty including all, we must add, which the Lord cites.
But two things make the difference. First it is law, not grace. Secondly, grace (though sanctioning all that) changed the relationship. We are God's children in grace, not Adam's. Though in the relationship, the obligation remains. Hence duty and measure of duty flowing from, and existing in, the relationship, that rule and measure are different. Grace says not, “Man must love God” (right, perfectly right as this is as a matter of duty, though hopeless for man in form), but “God loves” and “we love” (not “must” love) “Him because He first loved us.” It is a new nature, and a new relationship. J.N.D.