Carolyn Arends gets it right in her piece at CT:

The story of Christianity ultimately leaves me shocked at the risks God takes with humans. Even the greatest lights in church history were dishearteningly imperfect. For all his heroism, Luther attacked the Jewish faith so polemically the Nazis later misappropriated his writings for their anti-Semitic cause. Reformer Ulrich Zwingli advocated justification by faith and concern for the poor, but he also endorsed the executions of two of his brightest disciples because they became Anabaptists. Simons was an inspired Anabaptist leader, but he overzealously excommunicated many who did not live up to his pious standards.

Yet God did great things through these flawed people, much as he did with Abraham, Isaac, Peter, and Paul. As long as there is a human element in his church, it will be prone to corruption. But as long as his Spirit moves, there will be reform and renewal.