Major sins are called “kabair”. Some of them are:
Shirk (joining others with Allah), homicide, unlawful sexual relation, slandering someone with unlawful sexual relation, fleeing from the battlefield, sorcery, seizing the possessions of orphans wrongfully, treating parents unkindly, violating rights of people (rightful due), committing sins during pilgrimage, charging interest, stealing, gambling and consuming intoxicants.
If a Muslim commits a major sin, without underestimating it and having faith in heart, it doesn’t make that Muslim an ignorer of God (kafir).
The greatest and the most important one of the major sins is relegating Allah and His laws to second place and placing another being, which allegedly have some characteristics that are exclusive to Allah, to the primary place in one’s life. This is called shirk, joining others with Allah. It is called akbar al-kabair (the greatest sin).
“Surely, Allah does not forgive joining others with Him, but He forgives what is less than that for the one who follows His rules. Who joins others with Allah, certainly slanders Him with a tremendous sin.” (An-Nisa/ The Women, 4:48)
Dividing sins into two categories such as major sins and minor sins is evident by verses and hadiths. Related verses are:
“If you stay away from the major ones which you have been forbidden, We shall conceal your evil deeds (minor sins) and let you enter an honorable abode.” (An-Nisa/ The Women, 4:31)
“And those (believers) avoid the major sins and immoralities; and when they are angry, they forgive.” (Ash-Shura/ The Consultation, 42:37)
“Those who avoid the major sins and immoralities, only [committing] slight ones. Indeed, your Lord is vast in forgiveness. He was most knowing of you when He produced you from the earth and when you were fetuses in the wombs of your mothers. So do not claim yourselves to be pure; He is most knowing of who fears Him.” (An-Najm/The Star, 53:32)
“Say, ‘Come, I will recite what your Lord has prohibited to you: Do not join (equate) anything with Him, and keep doing good to your parents, and do not kill your children out of the fear of poverty; We provide for you and them. And do not approach immoralities – what is apparent of them and what is hidden. And do not kill the soul which God has forbidden except by [legal] right. This has Allah instructed you so that you may understand.
And do not approach the orphan’s property except in a way that is best [intending improvement] until he reaches maturity. And give full measure and weight in justice. We do not impose upon anybody a duty but to the extent of its ability. And when you speak be just, even if [it concerns] a near kin. And the covenant of God fulfill. That then He has charged you with; haply you will remember.
And, this is My path, which is straight, so follow it; and do not follow other ways, since this will separate you from My way. This has He charged you with that you may protect yourselves. ” (Al-An’am/ The Cattle 6:151-153)
“And your Lord has decreed that you not worship except Him, and do good to parents. Whether one or both of them reach old age [while] with you, say not to them [so much as], “uff,” and don’t be unconcerned to them but speak to them generous words.
And take them under the wings of mercy and say, “My Lord, have mercy upon them as they brought me up [when I was] small.”
Your Lord is most knowing of what is inside yourselves. If you should be righteous, He will indeed forgive those who return (to righteousness from mistakes).
And give the relative his right, and the desperate and the traveler, but do not spend wastefully.
Indeed, those who squander wastefully are brothers of the devils, and Devil is ever ungrateful to his Lord.
And if you turn away from them awaiting mercy from your Lord which you expect, then speak to them words that ease.
And be neither too tight-fisted nor wasteful, or you will sit by reproached and destitute.
Indeed, your Lord extends provision for whoever exerts the effort for it and complies with His measures. Indeed He is aware of (the insides) of His servants and He sees all.
And do not kill your children for fear of poverty. We provide for them and for you. Indeed, their killing is ever a great crime.
Do not approach fornication. It is definitely a shameful act and an evil way.
And do not kill the soul which God has forbidden, except by right. And whoever is killed unjustly – We have given his/her protector the authority, but let him not exceed limits in taking life. Indeed, he has been helped.
And do not approach the possessions of an orphan until he reaches maturity, except in a way that is better. And keep the promises you made. Indeed, promises bring responsibility.
And give full measure when you measure, and weigh with a straight balance. That is good and gives the best result.
And do not blindly follow that which you have no knowledge. Indeed, the hearing, the sight and the heart will be held responsible for it.
And do not walk upon the earth exultantly. Indeed, you will never tear the earth apart, and you will never reach the mountains in height.
All that aforementioned – evil is ever, in the sight of your Lord, disliked.
That is from what your Lord has revealed to you, [O Muhammad], of wisdom. And, [O mankind], do not make up another deity besides God, or you will be thrown into Hell, reproached and rejected.” (Al-Isra/The Night Journey 17:23-39)
It is reported on the authority of Abu Huraira that the Messenger of Allah observed:
“Avoid the seven noxious things.
It was said (by the hearers): What are they, Messenger of Allah?
He replied: Joining anything with Allah, doing witchcraft, taking the life of someone which Allah made inviolable without a just cause, consuming the property of an orphan, doing usury, turning back when the army advances and slandering chaste women who are believers, but unwary.” (Muslim, Sahih, Faith 145 (89))