You're right, 'Creer' is to 'Belive' and 'Pensar' is to 'Think'. For giving opinions both could be correct, and also to make predictions. But we normaly use 'Creo' for that. i
Also, when you are talking about specfici belifs (religious for example), then only 'creo' is correct. And if you are talking about the action of thinking about something, then only 'pienso' is correct.
If you belive in Jesus, you say 'Creo en Jesus'. If you are thinking about what to do for lunch, then you say 'Estoy pensando que hacer para almozar'.