I'm not sure if "documentary" is the right genre for this film, but documentation of the Batman comics is covered as well as that of how the New Testament was written and compiled. To show the parallel, the filmmakers get the people from the convention to sit down and determine which books, movies and shows from the entire history of Batman should be considered "official". In Biblical terms, this is called the "canon".
The movie then switches back and forth from the discussion about what the "real" Batman is to the history of how the New Testament was compiled. The discussions get quite passionate. The brief forays into history are well done and accurate and since you are seeing the parallels, entertaining. Also mixed in is a future scenario when Batman is a religion and people are debating if he was a real person. These don't just poke fun at those who would believe such a ridiculous thing, but also those who make bad arguments against it and then pat themselves on the back for being the smart ones.
It's all a lot of fun and such a relief from the spate of "Jesus is a myth" documentaries that have come out over the last couple decades. I think this one would stand up well to any scholarly historical peer review. It even throws in a little double-cross mystery at the end. I enjoyed this movie so much, that near the end, I needed to go the bathroom, and was seriously trying to figure out how I could do that without leaving the theater.