Judeo-Christians are known as people of the book.
I mean, Jews are
widely know by this title.
But I have heard preachers refer to Christianity as a whole in this way too.
Of all things, why a book?
Instinct might have also told them that there is something deeply Christian (and Jewish) about the act of reading. Both Christian and Jew are, and always have been, People of the Book. When God chose to give us His law, He didn’t draw pictures.