What's His Is Ours
It is because Luther was so strongly convicted of the presence of Christ in faith that he loved the marital imagery of Bernard and the Bible. In a sermon from early in his career (probably 1519) called "Two Kinds of Righteousness," Luther preaches: "Just as a bridegroom possesses all that is his bride's and she all that is his—for the two have all things in common because they are one flesh—so Christ and the church are one spirit." In this joyful exchange, our sin with its consequent death and damnation becomes Christ's, while Christ's righteousness becomes ours (1 Cor. 1:30). Only after we have received Christ's "alien righteousness," as Luther called it, do we begin to get somewhere with our own "proper" righteousness.
For Luther, a God demanding good works as the condition for our salvation would be as bad as the groom in the limo. How could we ever know that we had done enough? How much righteousness is needed to get us across the boundary line from damnation to salvation? And how can we be expected really to love and trust a God always watching us like a hawk to see if we fail? Promises to help us can only ring hollow. In reading Paul's letters over and over, Luther finally got it: Righteousness is not something that we generate from within ourselves and then offer to God. Righteousness is something that God gives to us—and this is exactly why we may trust in and grow to love God.
But Luther didn't expect that getting righteousness as a gift (instead of performing it ourselves) would have no effect on us at all. Certainly he couldn't imagine that we'd become like the callously indifferent bride, basking in the benefits of marriage but promising neither love nor devotion in return. Instead, our "soul no longer seeks to be righteous in and for itself, but it has Christ as its righteousness and therefore seeks only the welfare of others." In other words, it is only when good works are removed from the question of salvation—and when righteousness is a gift from God instead of a payment from us—that we can truly start performing them. They matter; not to God, since they aren't needed to settle the scales of divine justice anymore, but to our neighbors, the very people and the earth itself that God gave us in the gift of creation, even before the gift of righteousness.
By Grace or By Faith?
Being made righteous, being justified, is an act done to us by God, not something we enact ourselves. But where does faith come into it? Why does Luther insist that we are justified by faith—and why is it not enough to say that we are justified by grace? It is often thought that "justification by grace" would be the more accurate phrase, since it puts the emphasis on God's doing instead of ours. "Justification by faith" seems to suggest that actually, in the end, it's up to us to have faith: Salvation by good works has slipped in through the back door, faith being the ultimate good work.