The verbs pensare, credere, and ritenere have similar meanings. However, are they always interchangeable?
Pensare is "to think" and credere is "to believe", but ritenere seems like 1. it is a word for a higher register, perhaps more formal and literary, and 2. is used for when you are referring to belief in principles rather than just plain thought.
Take for instance the sentence: "penso di usare la rete ogni giorno". This isn't a problematic sentence. But the sentence, "ritengo che lui usi il telefonino ogni cinque minuti" sounds pretentious. The expressed thought is also not a deep conviction, it is a simple thought, so the use of ritenere here doesn't seem correct.