What sources does Martin Luther use to justify his argument?

What sources does Martin Luther use to justify his argument?
1. a 20-word (no more!) thesis in bold at the top, directly answering the
question (HAS TO BE EXACTLY 20 WORDS)
2. One or two paragraphs (approximately 300-350 words) of
argumentation and analysis supporting the thesis. It may be helpful to
use one or two brief quotes from the reading here
3. One question that you have after the reading (either a point of
clarification or a topic of discussion)