"What do you believe?"
Nah. It should be correct, though in modern speech, they only use the pronoun only once when making a new topic. So if you were giving someone an interview and you ask him what he believes, all you'd need to say is "Cosa credi?" But if you're a Christian missionary and you walk by someone's doorstep to ask them what they believe in, you'd say "Cosa credi tu?" since you haven't met them.