Maybe, sometimes, when someone says they believe a thing, even a thing we ourselves think of as obviously and intuitively incorrect, they are just being honest and they really do believe.
Perhaps the better approach, assuming you know what they believe at all, is to ask why they believe it and take the conversation from there.
Maybe you could even set up some kind of quid pro quo dialogue where we tell each other what we believe and why and see if there is any common ground between our ideas or legitimate criticisms, of either idea to be sure, but most especially our own if we do in fact want to have strong well considered beliefs.
Perhaps this is as good a place as any to start that conversation so if you aren't sure about someone's point if view involved, by all means starting with mine, please ask them.