Michael Reeves’ summation of Martin Luther’s The Freedom of a Christian:
Loved from Harlotry to Royalty: “At the heart of [The Freedom of a Christian] is a story of a king who marries a prostitute, Luther’s allegory for the marriage of King Jesus and the wicked sinner. When they marry, the prostitute becomes, by status, a queen. It is not that she made her behavior queenly and so won the right to the king’s hand. She was and is a wicked harlot through and through. However, when the king made his marriage vow, her status changed. Thus she is, simultaneously, a prostitute at heart and a queen by status. In just the same way, Luther saw that the sinner, on accepting Christ’s promise in the gospel, is simultaneously a sinner at heart and righteous by status. What has happened is the ‘joyful exchange’ in which all that she has (her sin) she gives to him, and all that he has (his righteousness, blessedness, life and glory) he gives to her. Thus she can confidently display ‘her sins in the face of death and hell and say, “If I have sinned, yet my Christ, in whom I believe, has not sinned, and all his is mine and all mine is his”.’ This was Luther’s understanding of ‘justification by faith alone’, and it is in that security, he argued, that the harlot actually then starts to become queenly at heart.”
 Michael Reeves, The Unquenchable Flame: Discovering the Heart of the Reformation (Nashville: B&H Academic, 2009), 50.