Some major sins regarding which severe warnings have been mentioned
To act contrary to one’s promise.
To betray a trust (amaanah).
To discard any fardh (compulsory duty) enjoined by Allah Ta’ala, e.g. salaat, fasting, hajj, zakaat, etc.
To forget the Holy Quran after having memorized it.
To speak lies. Especially to take false oaths.
To swear by the name of anyone other than Allah.
To take an oath with such words that the person is deprived of the kalimah at the time of death, or may die without Imaan.
To prostrate before anyone other than Allah Ta’ala.
To miss salaat without a valid excuse.
To call any Muslim a kafir, or non-believer, or to invoke the wrath or punishment of Allah on him, or to call him an enemy of Allah.
To complain against someone, or to hear such a complaint.
To take interest (usury).