I want to test the ability of students to read a passage and answer questions about it. Could you please come up with a good question for the passage "Luther secretly returned to Wittenberg on 6 March 1522. He wrote to the Elector: "During my absence, Satan has entered my sheepfold, and committed ravages which I cannot repair by writing, but only by my personal presence and living word." For eight days in Lent, beginning on Invocavit Sunday, 9 March, Luther preached eight sermons, which became known as the "Invocavit Sermons". In these sermons, he hammered home the primacy of core Christian values such as love, patience, charity, and freedom, and reminded the citizens to trust God's word rather than violence to bring about necessary change."? 

It is implied that Martin Luther preached his eight sermons in response to the use of what by Christians?