"Unicorns exist" and "unicorns don't exist" are both factual statements of which one can have a belief. Right now you probably hold one or the other. Sure, if you'd never heard of unicorns, such a thing wouldn't enter your head; but you
have heard of unicorns, and thus you
do have an opinion on their existence.
Similarly, if one lived all one's life in a rationalist bubble, and never even heard the mention of God or gods or religion or the supernatural, then perhaps one could not have an opinion on whether God exists. But that applies neither to you nor to Dennet.
How would I know that my disbelief in Thor is wrong? At a first cut, I'd need to have someone propose a more concrete proposition to evaluate; then I could try to evaluate it. But whatever that proposition is, it would need to be able to accommodate all that we've learned about the world and about science; it would need to be falsifiable; and it would need to explain the world in a more satisfactory manner than the alternative worldviews.
Similarly, if one lived all one's life in a rationalist bubble, and never even heard the mention of God or gods or religion or the supernatural, then perhaps one could not have an opinion on whether God exists. But that applies neither to you nor to Dennet.
How would I know that my disbelief in Thor is wrong? At a first cut, I'd need to have someone propose a more concrete proposition to evaluate; then I could try to evaluate it. But whatever that proposition is, it would need to be able to accommodate all that we've learned about the world and about science; it would need to be falsifiable; and it would need to explain the world in a more satisfactory manner than the alternative worldviews.