If thou wert in doubt as to what We have revealed unto thee then ask those who have been reading the Book from before thee: the Truth hath indeed come to thee from thy Lord: so be in no wise of those in doubt. [Quran, Yunus 10:94, Yusuf Alis translation]".
This verse was the key of answering all my questions and removing all my doubts
so I wondered, who are those who were reading "the Book" before the Muslims?
The answer came to me from the Quran itself! Those are the Jews and Christians for the Quran calls them "the People of the Book" because they have the Torah and the Injeel (Gospel), and the Quran orders every Muslim who doesnt understand it to go back to the Torah and the Injeel (Gospel).