[W]hy the Lord says, “He that doeth the things of the law shall live,” considering no man since the fall can do the things of the law? Answer. The Lord since man’s fall repeats the law in his [its] old tenor, not to mock men, but for other weighty causes. The first is to teach us that the law is of a constant and unchangeable nature. The second is to advertise us of our weakness and to show us what we cannot do. The third is to put us in mind that we must still humble ourselves under the hand of God, after we have begun by grace to obey the law, because even then we come far short in doing the things which the law requires at our hands.
William Perkins (1558–1602), Commentary on Galatians, in Works, 2:178. (HT: Inwoo Lee)