believe that something is ...; believe something to be ...
→ だ
phraseCommon#6,164
believe that something is ...; believe something to be ...Used after a noun or な-adjective followed by だ to express belief that something is a particular identity, state, or quality. In polite or progressive forms, it may appear as だと信じています or だと信じている. With verbs and い-adjectives, use the plain form directly before と信じる instead.