The word ‘book’ or kitab in Islam has a very broad meaning. It refers to the Koran as the verbatim word of God, but this Koran itself one ‘book’ and the same term is used for all sacred scripture, which is why, after becoming familiar with the Hindu doctrines, Islam accepts Hindus as ‘People of the Book’. This is because, according to the Koran, there is in Heaven the ‘archetypal book’ which is itself the source of all particular revelations. This is the Umm al-kitab, the Mother Book. This is why all authentic revelations convey basically the same message, but in the language of the people to whom the message is given, so that they may understand it: “We never sent a messenger save with the language of his people.”
Thus, much like Catholicism when it speaks broadly of ‘the Church’ as the global community of believers, when the Koran says that the religion of God is al-islam, it is speaking of the universal doctrine of submission to God and His Oneness that all religions possess, and in this way Muslims are well-situated to show respect to members of other religious traditions.
 Koran 14:4.