Use Credo to get background information on your topic by searching for people, places, and historical events.
Luther's Works is a large set of books that contains Luther's sermons, commentaries, theology, etc. Use the table of contents listed in each link below to help you determine which volume you would like to consult.
Cambridge Dictionary of Christian Theology by 
Global Dictionary of Theology by 
Encyclopedia of Ancient Christianity, Volumes 1-3 by 
The Oxford Encyclopedia of the Reformation, volumes 1-4 by 
