This article seeks to uphold a consistently legal reading of Luther’s conception of temporal authority. Far from a premature dismissal or milieu-motivated relativization of the reformer’s precepts, it shows that such an analysis need degenerate neither into casuistry nor naïve pre-Enlightenment authoritarianism. Rather, I argue that what drives Luther’s esteem for temporal authority—which he views primarily in light of its social and vocational expression in civil law—is the ancient legal maxim that no one be judge in one’s own case, nemo iudex in causa sua. On this basis Luther proposes a noncasuistic theory of the law and, in so doing, destabilizes the relation between the Christian and temporal authority while at the same time keeping at bay the threat of self-serving individualism and anarchy. I argue, moreover, that the maxim underlies Luther’s conception not only of the political use of the law but also, by exposing its shortcomings, of the law’s theological, accusatory, function. As such, to push the argument further still, I also propose that the maxim may be viewed as the very foundation of Luther’s mature understanding of justification and Christian life. God reveals that he is a righteous judge by consistently not being judge in his own case and so being free to justify humanity. In keeping with the maxim, this in turn makes the believer, as a justified sinner, uniquely able to uphold civil law with a view to according the neighbor justice. In doing so, the believer actually justifies God, who has first justified the believer.