Five of the criteria offered by scholars for major sins are as follows:
1- Any sin for which God, in the Quran, has promised punishment.
2- Any sin for which Islam has considered a Hadd, such as drinking wine, fornication, etc.
3- Any sin that shows scorn of religion.
4- Any sin that is proven with clear evidence to be Haram and grave.
5- Any sin that the Quran and Sunnah have severely warned those committing it.
As for the number of major sins, there have been different numbers mentioned, including 7, 10, 20, 34 and 40.
According to the book Tahri al-Waisla by Imam Khomeini (RA), the major sins are many, and they include:
1- Losing hope in God’s mercy
2- Feeling secure from the retribution of God
3- Attributing untrue statements to God, the Prophet (PBUH) and the Infallibles (AS)
4- Murder
5- Disrespecting parents
6- Consuming the properties of orphans
7- Defaming chaste women
8- Running away from war front
9- Severing family ties
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10- Sorcery and witchcraft
11- Fornication
12- Sodomy
13- Theft
14- Taking false oath
15- Concealing testimony
16- Giving false testimony
17- Breaking promise
18- Acting against someone’s will
19- Drinking wine
20- Usury
21- Consuming properties obtained through Haram means
22- Gambling
23- Eating animal corpses and drinking blood
24- Eating pork
25- Eating the meat of an animal that has not been slaughtered in accordance with religious law
26- Short weight
27- Migrating to a place where one will lose his religion
28- Helping oppressors
29- Relying on oppressors
30- Keeping for oneself what is others’ rights
31- Lying
32- Haughtiness
33- Wastefulness
34- Malversation
35- Backbiting
36- Gossiping
37- Engaging in diversion and too much amusement
38- Failing to attach importance to Hajj
39- Abandoning Salah
40- Not paying Zakat
41- Insisting on committing minor sins.
And of course, Shirk (polytheism), denying what God has commanded, and enmity with men of God are among gravest sins.