Learn Italian: Il magistrato ha ascoltato l'imputato, ma non gli è stato creduto. - The magistrate listened to the defendant, but he was not believed.

In English, 'he' is the subject of the passive verb 'was believed'. But in Italian, 'to believe a person' is credere a qualcuno, which takes an indirect object. Verbs that take an indirect object cannot form a normal personal passive where the person becomes the subject. Instead, Italian uses an impersonal passive: è stato creduto (literally 'it was believed') combined with the indirect object pronoun gli (to him).

Since creduto is a past participle used with essere, does it need to agree with the pronoun gli?

No, the past participle remains in the masculine singular form. In this impersonal passive structure, gli is an indirect object, not the subject. The grammatical subject is an invisible, impersonal 'it' (literally 'it was believed to him'), so the past participle simply takes the default masculine singular ending. Even if the defendant were female, it would be le è stato creduto, still ending in -o.

The first half of the sentence treats the defendant as a direct object (ha ascoltato l'imputato). Why does the second half switch to an indirect object?

The grammar depends entirely on the verb being used. Ascoltare (to listen to) is a direct transitive verb in Italian, so it takes a direct object. However, credere (to believe) requires the preposition a when you are talking about believing a person. Therefore, when referring back to the defendant for the second verb, you must use the indirect pronoun gli.

Could I use the passive here instead, and say ?