Why does Paul call the law weak and beggarly from humanity's standpoint?
From humanity's standpoint, the law was weak and beggarly because it was powerless to get the job done. It could only point the finger of condemnation at mankind. Although the law itself is perfect, holy, and righteous, it could not produce righteousness in man; the covenant must begin and end with God.
Les explains that the Apostle is speaking as a human, after the manner of men, and recognizes that as far as humanity is concerned the Law was weak and beggarly. It simply could not get the job done. This sometimes shocks people because we ordinarily think of the Law from the other side—that it was perfect, holy, and righteous—and it is. But from the human standpoint it was powerless, and all it could do was point the finger of condemnation at mankind. Yet because it is a covenant, and every covenant in Scripture is a one-way street, it begins and ends with God. Men cannot touch it, enlarge on it, subtract from it, or add to it. Even though from man's standpoint it is weak and beggarly, once confirmed no man can disannul it or add to it.
Brethren, I speak after the manner of men; Though it be but a man’s covenant, yet if it be confirmed, no man disannulleth, or addeth thereto.