Thanks to his gift for pithy and salty expression and his passion for transgressing linguistic and social boundaries, Martin Luther makes it possible for a biographer with some ease to invite into his world people who might, in the normal course of things, stand outside it. It is the biographer’s task to make them feel sufficiently at home in that world that they can make judgments about the story and sufficiently ill at ease in that the telling can provoke them into fresh thinking.

In this quote from the introduction of Martin Luther, Marty explains that his biography will be accessible to an ordinary, non-theological audience. The biography is laid out in a traditional, chronological fashion and includes many of the events that flesh out Luther's life and personality, as well as providing a sense of Luther's times. As Marty explains, he approaches his subject as a historian and not a theologian, and he wants his readers to be able to make up their own minds about Luther while thinking about him in new ways.

Luther ... began a lifelong search for ways in which humans could experience the love of God without using God, without turning God into a convenience.

Early in the book, Marty discusses ways in which Luther's thinking differed from the Roman Catholic Church. In this quote, Marty is differentiating between the Catholic idea of contrition, in which believers worked to prove they could please God, and a stance of accepting God's love and grace as flowing freely—not earned through works, but responded to by works.

In the matter of being declared just, the law of God always and only accused sinners, which meant it judged everyone in the church.

Luther agreed with the traditional theology of sin but saw in faith, rather than works, a road to forgiveness and redemption.