An infinitive is the word to followed by the base form of a verb. Subjects of infinitives are an exception to the rule that subjects must be in the subjective case. Whenever an infinitive has a subject, it must be in the objective case. Objects of infinitives also are in the objective case.
The pronoun whom is the subject of the infinitive to believe.
Exercise: Pronoun case (who and whom) 1
Exercise: Pronoun case (who and whom) 2
Exercise: Pronoun case: review 1
Exercise: Pronoun case: review 2
Related topic:
Infinitive phrases
base form The dictionary form of a word, without any prefixes or suffixes or markers of tense or case.