Q.The individual is asking how do we know what is a major sin?
A. The shaykh said major sins are for example that which Allaah tabaarak ta wa ta’aala, informs us the punishment for whoever performs it will be the hell fire. Likewise that which the Prophet (Sallaahu alayhi wa sallam) informed us that whoever does this action they will be distant from the mercy of Allaah, or whoever does this action will be cursed by Allaah. These are certain things which inform an individual that a sin is a major sin.
You have the hadith where the Prophet (Sallaahu alayhi wa sallam) said: ‘abstain from the seven major sins’. However the Prophet Sallaahu alayhi wa sallam did not mean that there are only seven. He sallaahu alayhi wa sallam in that hadith mentioned seven but did not say that they are the only major sins. The proof for that is ibn Abbass (RadiAllaahu anhumma). He was asked are there only seven? He said they are closer to seventy then seven. So there is no specific number of the amount of major sin there are.
Shaykh Saalih ibn Muhammad al-Luhaydaan (May Allaah preserve him) (Kitaab-ut-Tawheed Class 01.12.12) Riyadh. Translated by Mustafa George hafidhahullaah