1. The first thing to know is that there are no good works except those works God has commanded, just as there is no sin except that which God has forbidden. Therefore, whoever wants to know what good works are as well as doing them needs to know nothing more than God’s commandments. Thus in Matthew 19[:17] Christ says, “If you would enter life, keep the commandments.” And when the young man in Matthew 19[:16–22] asks what he should do to inherit eternal life, Christ sets before him nothing else but the Ten Commandments. Accordingly, we have to learn to recognize good works from the commandments of God, and not from the appearance, size, or number of the works themselves, nor from the opinion of men or of human law or custom, as we see has happened and still happens because of our blindness and disregard of the divine commandments.
—Martin Luther, Luther’s Works, ed. Jaroslav Jan Pelikan, Hilton C. Oswald, and Helmut T. Lehmann (Philadelphia: Fortress Press, 1999), 44.23.