"[O you who have believed, believe in Allah and His Messenger and the Book that He sent down upon His Messenger]."

Al-Kalbi said: He addressed with this those among the People of the Book who believed; and that is because they said upon their conversion to Islam: "Should we believe in the Book of Muhammad and disbelieve in what is other than it?"

So Allah said: "[Say, 'Believe in Allah and His Messenger and the Book that He sent down upon His Messenger']," until the end of the verse.