Poisoned Pope
Secrecy is nearly synonymous with the inner workings of the Vatican, and they certainly want to keep this story hidden if it happens to be true. This conspiracist is under the belief that Pope John Paul I did not die of natural causes. Instead, they claim that he was assassinated in an attempt to keep some Vatican secrets hidden.
The reason assassination was necessary, according to the theory, is that Pope John Paul I was attempting to uncover corruption in the Vatican bank. To prevent that uncovering, he was poisoned and cremated to cover up the evidence. Considering he is the only cremated pope in history, some aspects of the story do seem questionable.