In his ‘Ninety-Five Theses’ Luther called upon believers to repent. What does that mean?

Reformation season is a time for much accusing of ancestors, from Columbus to Thomas Jefferson, now remembered as slavers, or, to be relevant, Luther, for his call for violence against rebelling peasants or his utterly, utterly repugnant anti-Judaic latter-day outlook and writings. We historians study such features of the lives of ancestors, to learn and gain the resolve to promote a “change of heart.”