If anything, fiction is one of the best ways to manipulate people's beliefs, because people believe that they are not being asked to believe things about the real world, but they are.

They are, because all fiction is based on selecting a common background with the real world, and then adding some unreal elements to that background. So for example you have historical fiction, where the background includes many real world historical events, or facts about a certain period. Or you have contemporary fiction, where includes facts about how the world happens to be at the moment. Or you have science fiction, which possibly includes various scientific laws, or at least obvious facts such as the claim that many bodies are solid, and so on.

The problem is that the reader doesn't go through line by line and distinguish between elements that are in common with the real world and ones that are not. He does this subconsciously. This means that he may often mistake something for an element of the real world, when it is not, simply because the author presents it as though it were part of the common background. The reader isn't in the mood for thinking about what is a fact and what isn't, and anyway he may not know. He just subconsciously absorbs it.

To give one minor example, in one of Dan Brown's books, someone ends up crippled for life, and a priest says, "He must not have had enough faith." Obviously this is supposed to be fiction. But if you present a priest saying this, most people would assume that the common background is that this is the kind of things that priests say. If you are not familiar with Catholic priests, you might end up believing that this is a credible thing for a Catholic priest to say, even without thinking about it explicitly. But basically no priest would say such a thing, so you have ended up believing something false.