The chairman, along with the team, _______ in your idea.
Word like with, together with, along with, besides, as well as, including, in addition to, etc. do not affect the number of the verb. If the subject is singular, a singular verb is required; if plural, a plural verb. The presence of ‘along with the team’ does not affect the quantity of the subject, ‘Chairman’ (singular). Thus, singular verb ‘believes’ will be used. NOTE: Belief is a noun and not a verb. Thus, it is not the appropriate choice.