Your subject would be the Marburg Colloquy, but your thesis has to be an arguable point.
In fact, that's the definition of a thesis : 1. an unproved statement put forward as a premise in an argument; 2. a treatise advancing a new point of view resulting from research.
You might argue something like, "The Marburg Colloquy is the greatest thing since sliced gefilte fish; everybody ought to be on board." Then set about proving your point. Mere historical reporting doesn't cut it.