Based on Al-Kabair (The Major Sins) by Imam al-Dhahabi, and authentic hadith from Sahih al-Bukhari, Sahih Muslim, and other collections.
Shirk — Associating Partners with Allah
Worshipping anyone or anything alongside Allah, or attributing divine qualities to creation. This is the gravest sin and the only sin Allah has declared He will not forgive if one dies upon it.
Quran 4:48 — "Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills."
Apostasy — Leaving Islam
Renouncing the religion of Islam after having accepted it. This is among the gravest violations as it severs one's covenant with Allah.
Quran 3:85 — "And whoever desires other than Islam as religion, never will it be accepted from him."
Hypocrisy — Nifaq
Outwardly professing Islam while inwardly rejecting it — showing faith to people while concealing disbelief in the heart.
Quran 4:145 — "Indeed, the hypocrites will be in the lowest depths of the Fire."
Magic and Sorcery
Practicing or seeking witchcraft, black magic, fortune-telling, or consulting soothsayers. The Prophet ﷺ listed it among the seven destroyers.
Sahih al-Bukhari 2766 — "Avoid the seven destroyers... magic..."