This is because Allah has revealed the Book with the truth; and surely those who go against the Book are in a great opposition.

(All) people are a single nation; so Allah raised prophets as bearers of good news and as warners, and He revealed with them the Book with truth, that it might judge between people in that in which they differed; and none but the very people who were given it differed about it after clear arguments had come to them, revolting among themselves; so Allah has guided by His will those who believe to the truth about which they differed and Allah guides whom He pleases to the right path.

Then there came after them an evil posterity who inherited the Book, taking only the frail good of this low life and saying: It will be forgiven us. And if the like good came to them, they would take it (too). Was not a promise taken from them in the Book that they would not speak anything about Allah but the truth, and they have read what is in it; and the abode of the hereafter is better for those who guard (against evil). Do you not then understand?