You see that the First Commandment, which says, “You shall worship one God,” is fulfilled by faith alone. Though you were nothing but good works from the soles of your feet to the crown of your head, you would still not be righteous or worship God or fulfill the First Commandment, since God cannot be worshipped unless you ascribe to him the glory of truthfulness and all goodness which is due him.
This cannot be done by works but only by the faith of the heart. Not by the doing of works but by believing do we glorify God and acknowledge that he is truthful.
Therefore faith alone is the righteousness of a Christian and the fulfilling of all the commandments, for he who fulfills the First Commandment has no difficulty in fulfilling all the rest.
Martin Luther, On Christian Liberty (Fortress Press, 2003), 22.