Martin Luther is the first and primary character in Martin E. Marty's biography, Luther. Young Luther graduated from the university at Erfurt and made an independent decision to become a monk. Luther's father, Hans, was totally surprised at his son's decision and objected to it. Luther, however, went ahead and joined an Augustinian monastery at Erfurt and became a priest in 1507. After Luther's first celebration of Mass attended by his father, father and son clashed—Hans said that his son's choice to become a priest was against the Fourth Commandment: "Honor thy father and thy mother."

Johann von Staupitz, Luther's superior at Erfurt, appointed him acting professor at the university at Wittenberg—a prestigious position for the young priest. Three years later, von Staupitz sent Luther to Rome. This was a pivotal moment in Luther's life, since in Rome he saw a dirty, corrupt city and notable lack of piety among the Vatican priests. It began what was to be his life's work: the Reformation of the Church.

He returned to Wittenberg, where he taught for most of the rest of his life. Nearby, a priest named Johannes Tetzel was infamous for the practice of selling indulgences, which were basically tickets to forgiveness of sin. Prince Frederick the Wise had an extensive collection of relics claimed to be from Jesus and Mary, which were dubious at best. In this, Luther saw a clear perversion of what the church ought to be.

In 1518, Luther famously penned a letter to the Archbishop, Ninety-Five Theses, criticizing the practices of the Catholic Church; he nailed it on the door of Castle Church. Within a few years Luther was excommunicated, but he had developed a large following. In 1522, Luther freed a convent of nuns near Wittenberg. He ended up marrying one of the nuns, Katherine von Bora, who bore him six children (two of whom died).

God has to be considered a prominent character in Luther, since Martin Luther's main message was that it was in prayer, reading the Bible, and personal contact with God that correct faith was to be found—not in the laws of a man-made church.