I was thinking about this yesterday and took note of some verses from Surah Al-Imran (3)…
“Not all of them are alike: of **the People of the Book **are a portion that stand (for the right); they rehearse the Signs of Allah all night long, and they prostrate themselves in adoration. They believe in Allah and the Last Day; they enjoin what is right, and forbid what is wrong…” (113-114).
“And there are, certainly among the People of the Book, those who believe in Allah, in the revelation to you, and in the revelation to them, bowing in humility to Allah: they will not sell the Signs of Allah for a miserable gain! For them is a reward with their Lord” (199).
According to what I’ve noticed while reading the Qur’an, the members of the People of the Book who believe in one God (Allah), His Signs, and the Last Day, are believers. The Unbelievers are those who pretend to believe in God or reject Him completely. ***I could be wrong though.