In 1520 Luther sought to put to rest the charges of antinomianism that were flying at him left and right, writing the 100-page booklet A Treatise on Good Works. The following paragraph from that work captures the heart of what Luther has been teaching me this year. It comes in the course of a discussion in which Luther is arguing that faith necessarily produces works, and that, conversely, no work is good unless it is founded on faith.
This is the work of the first commandment, which enjoins, 'Thou shalt have no other gods.' This means, 'Since I alone am God, thou shalt place all thy confidence, trust, and faith in me alone and in no one else.' For you do not have a god if you just call him God outwardly with your lips, or worship him with the knees or bodily gestures; but only if you trust him with your heart and look to him for all good, grace, and favor. . . . [T]his faith, this trust, this confidence from the heart's core is the true fulfilling of the first commandment. Without such faith no work at all can satisfy this command. And because this commandment is the very first of all commandments and the highest and the best, the one from which all others proceed, in which they exist and by which they are judged and assessed, so its work (that is, the faith or confidence that God is gracious at all times) is the very first, highest, and best from which all others must proceed, in which they must exist and abide, and by which they must be judged and assessed. (LW, 44:30)
Two critical insights emerge from this densely packed paragraph.
(1) What is the first commandment, with its prohibition of idolatry? A call to justification by faith. The only alternative to justification by faith alone is idolatry--justification by an idol. The OT says 'You shall have no other gods before me'; the NT says 'We hold that one is justified by faith.' It boils down to the same dynamic of the heart.
(2) Any breach of commandments 2-10 is necessarily at the same time a breach of commandment number one. To commit adultery, e.g., is not simply to break a rule; it is to have another god (sex) before Yahweh. It is justification by sex. The first commandment is the umbrella under which the others subsequently fall.