The greatest sin: Shirk!
(Associating/joining others with Allah in His Divinity or worship, Idolatry)
Allah is closer to us, than our jugular vein (Qáf 50:16). But shirk alleges that Allah is far and insufficient. Shirk is:
• To want someone as an intermediary or occasion instead of Allah (Al-Ahqáf 46:5; Az-Zumar 39:3).
• To attribute to Allah children, wife, partner; to idolize someone or something (Al-An’ám 6:100; Al-Furqán 25:43). To worship someone else other than Allah (Al-Ìmrán 3:64) and get intercessors (Al-Anám 6:94; Yunus 10:18).
• To regard someone as a haram/halal legislator and authority in religion instead of Allah (Al-Ìmrán 3:80; Al-Anám 6:114, 148-150; Al-Kahf 18:26; An-Nahl 16:35; Ash-Shura 42:21; At-Taubah 9:31; Yusuf 12:40).
• To pray and call someone to assistance who is dead instead of Allah (Al-Jinn 72:18; Fatir 35:13-14).
• To like or count anything equal or more than Allah (Al-Baqarah 2:165; At-Taubah 9:24).
Even Prophet Abraham and Mohammad were warned about Shirk (Al-Hajj 22:26; Az-Zumar 39:65). Allah, told that he will not forgive someone who commits shirk and does not repent it (Al-Máidah 5:72; an-Nisa 4:48, 116) and that
everything that he did in his life will be wasted (Az-Zumar 39:65). With shirk begins exploitation and slavery. Shirk is an injustice against Allah (Luqman 31:13).