When the verse goes down: "Believers and do not mix up their faith with injustice" the Companions of the Messenger of Allaah (peace and blessings of Allaah be upon him) said:

"Who among us is not doing zhalim??

So Allah 'azza wajalla decreed (His word)::

"Surely Syirik is great injustice".
(Luqman: 13)
(Bukhari 31)