It is crucial for a Muslim to have some understanding of the major (kaba'ir) and minor (sagha'ir) sins, what the major sins are, the difference between these two categories of sin and how ultimately to avoid them. A related question, which we will leave for a later discussion, is the reasons why people fall into sin in the first place. The quality and level of our spiritual life fundamentally depend on that knowledge.
An important point to bear in mind is that according to the Quran our minor sins will not be forgiven until and unless we refrain from the major ones. "If you refrain from the kaba'ir (major sins) you have been prohibited from, We will expiate your sayyi'aat (minor sins) and place you in the Jannah." [Surah al-Nisaa, v.31]. The Nabi, may the peace and blessings of Allah be upon him, clarified the intention of this verse when he said," The five daily prayers (salah) expiate whatever has been committed between them if the kaba'ir (major sins) are avoided". If we perform our daily salah one of the consequences is that our minor sins are forgiven on condition that we also refrain from the major sins. The hadith also implies that the solution to the major sins is repentance, and in some cases punishment (hadd), not the performance of five pillars of Islam. The major sins in fact nullify the ‘ibadah one may be doing. This is indicated in the verse, "O you who believe obey Allah and obey His Rasul and do not nullify your actions" [Surah Muhammad, v.33]. In other words do not nullify your good deeds by committing any of the major sins.
So the question is what then are the major sins?
Scholars have differed on the subject from the time of the Sahabah, may Allah be pleased with all of them. A very interesting opinion was expressed by Abdallah ibn Mas'ud, may Allah be pleased with him. He was asked about the meaning of verse 31 of Surah al-Nisaa, and he replied, "Read from the beginning of this Surah up to verse 31 then consider all the things Allah prohibits in those verses, major sins." He consequently believed there were only four major sins. Ibn ‘Umar, may Allah be pleased with him, said there are seven major sins. He based his judgement on the well-known hadith of the al-saba'ah al-mubiqat (the seven grave sins). Abdallah ibn "Amr may Allah be pleased with him, believed there where nine. Other early scholars preferred to define it as every sin that incurs a punishment (hadd). The minor sins they say are more like faults, an inadvertence and shortcomings which are neither punishable nor is there a promise of hell fire for the one who is guilty of them. And finally Abu Hurairah may Allah be pleased with, according to certain reports, said the major sins are hidden and their number unknown like laylah al-qadr and the blessed hour of jumu'ah.
Given all of these differences and the fact that knowledge of the major sins are crucial to the spiritual well-being of every Muslim later scholars decided to combine all of these views and opinions.
The result of this process is summarised by Abu Talib Makki in the listing he provides. [Qut al-Qulub, pg. 365 - 7, vol. 3]. He says " As to my own position in this matter after combining the different opinions is that the major sins are seventeen:
Four of the heart;
1 idolatary (shirk),
2. persistent transgression (israr),
3. loosing hope in the Mercy of Allah (qunut),
4. and to feel secure from the Divine Cunning (amn min makr Allah),
Four of the tongue;
5. bearing false witness (shahadah al-zur),
6. defaming a woman of good standing (qadhf al muhsan),
7. oaths that undermine justice (al-yamin al ghamus),
8. sorcery and black magic (sihr), this includes all the actions and words used to change the perceptions, bewitch, beguile and harm other human beings.
Three of the stomach;
9. drinking of wine and all kinds of intoxicants,
10. unjustly eating the wealth of orphans,
11. and consuming riba,
Two of the private parts;
12. fornication (zina),
13. and anal sex (liwat),
Two sins of the hand;
15. and stealing,
One of the feet;
16. running away from the battlefield when confronted by the enemy of the Muslims,
One of the entire body;
17. disrepect for one’s parents."