Still more from Carl Trueman’s article Luther’s Theology of the Cross:
Luther does not restrict the theology of the cross to an objective revelation of God. He also sees it as the key to understanding Christian ethics and experience. Foundational to both is the role of faith: to the eyes of unbelief, the cross is nonsense; it is what it seems to be—the crushing, filthy death of a man cursed by God. That is how the unbelieving mind interprets the cross—foolishness to Greeks and an offence to Jews, depending on whether your chosen sin is intellectual arrogance or moral self-righteousness. To the eyes opened by faith, however, the cross is seen as it really is. God is revealed in the hiddenness of the external form. And faith is understood to be a gift of God, not a power inherent in the human mind itself.
This principle of faith then allows the believer to understand how he or she is to behave. United to Christ, the great king and priest, the believer too is both a king and a priest. But these offices are not excuses for lording it over others. In fact, kingship and priesthood are to be enacted in the believer as they are in Christ—through suffering and self-sacrifice in the service of others. The believer is king of everything by being a servant of everyone; the believer is completely free by being subject to all. As Christ demonstrated his kingship and power by death on the cross, so the believer does so by giving himself or herself unconditionally to the aid of others. We are to be, as Luther puts it, little Christs to our neighbors, for in so doing we find our true identity as children of God.