Associating others with Allah is the gravest of major sins and the worst act of zulm (injustice, wrongdoing). It is an injustice that the one who associates others with Allah does to himself, because Allah has made clear to all people the path of guidance and the way to learn about Allah and His Oneness. In this hadith, ‘Abdullah ibn Mas‘ud (may Allah be pleased with him) narrates that when the verse {They who believe and do not mix their belief with injustice (zulm)} [al-An‘am 6:82] was revealed, the Companions of the Prophet (blessings and peace of Allah be upon him) were distressed by that, because they thought that what was meant by zulm was sin in general, as one might initially understand, especially because the Arabic word appears in the indefinite form, which would suggest a general meaning referring to any act of wrongdoing. They were worried about that because the apparent meaning of the word zulm is to transgress against people and not give them their dues, and to transgress against oneself by committing sins. Hence they thought that what was meant here was the apparent meaning that first springs to mind, which is doing that which is not appropriate and is contrary to Islamic teachings. So they were concerned, because no one is safe from falling into such sins. Then Allah revealed the words: {Indeed, association [with Him] is great injustice (zulm)} [Luqman 31:13]. Thus it became clear that what is meant is the worst type of zulm, which is shirk (associating others with Allah). Shirk is tantamount to zulm because Allah (may He be glorified) is the Bestower of all blessings, so if His slave associates anything else with Him, he has committed a grave wrong (zulm)..