1. It is believed.—
2. Who spoke.
Now, here, it is the subject, and, as such, nominative. But it is also the equivalent to to be he, which must be nominative as well. To be he is believed=esse-ille creditur,—or, changing the mode of proof,—
1. It is the subject and nominative.
2. Believed is part of the predicate; and, consequently, nominative also.
3. To be he is a subordinate part of the predicate, in apposition with believed—est creditum, nempe entitas ejus. Or, to be he is believed=esse-ille est creditum.
As a general expression for the syntax of copulas and appositional constructions, the current rule, that copulas and appositional verbs must be followed by the same case by which they are preceded, stands good.