The Happy Exchange
Luther's image (fröhlicher Wechsel) for what happens in faith, taken from marriage law: Christ takes the sinner's sin, death, and damnation, and the sinner receives Christ's righteousness, life, and blessedness. Everything each party owns becomes the other's. It is the engine of The Freedom of a Christian and of the hymn “Dear Christians, One and All, Rejoice.”
In Doctrine — What Lutherans teach, and where it differs from neighbouring traditions.