If you believe something, you think it’s true. If you are easily duped then you might believe your little sister when she says a thief broke into the house and ate all your candy.
Believe comes from old German words meaning to have trust. Belief doesn’t require proof, just acceptance. If you’re on a jury and the prosecution and defense present two different versions of what happened, you have to decide which side you believe. To believe can also mean having confidence. If you believe in me, it doesn’t mean that you think I’m real (unless I’m a ghost). It means you think I can do what I set out to do.