You've already admitted you lacked belief in magical unicorns made of meteorite teapots living in my dog's butt before you knew they existed, so you've already confirmed yourself wrong. Do you need to know what a merkin is to lack one? Do you need to know what a digeridoo is to lack one? In what sense is it required to know what something is to lack it?
Maybe you will understand if I turn it around.
Remember belief deals with different degrees of certainty, weal/strong belief etc.
Do you need to know what a merkin is to lack one?
Would you say you lack belief in that I have a dog if I told you I do?
Most people would probably have to say no.
Do they need to know what my proposition was to not lack belief in it?
Hence you need know what the proposition is to attribute a degree of belief to it.