What are the greatest sins?
The Prophet ﷺ named the greatest of the great sins: associating partners with Allah, being undutiful to parents, and false statements and false witness — he repeated this last warning many times. Allah does not forgive shirk if one dies upon it, but forgives anything less for whom He wills. The door of tawbah is open until death.
Evidence
إِنَّ ٱللَّهَ لَا يَغۡفِرُ أَن يُشۡرَكَ بِهِۦ وَيَغۡفِرُ مَا دُونَ ذَٰلِكَ لِمَن يَشَآءُۚ وَمَن يُشۡرِكۡ بِٱللَّهِ فَقَدِ ٱفۡتَرَىٰٓ إِثۡمًا عَظِيمًا
Indeed, Allāh does not forgive association with Him, but He forgives what is less than that for whom He wills. And he who associates others with Allāh has certainly fabricated a tremendous sin.
وَإِذۡ قَالَ لُقۡمَٰنُ لِٱبۡنِهِۦ وَهُوَ يَعِظُهُۥ يَٰبُنَيَّ لَا تُشۡرِكۡ بِٱللَّهِۖ إِنَّ ٱلشِّرۡكَ لَظُلۡمٌ عَظِيمٞ
And [mention, O Muḥammad], when Luqmān said to his son while he was instructing him, "O my son, do not associate [anything] with Allāh. Indeed, association [with Him] is great injustice."
أَلاَ أُنَبِّئُكُمْ بِأَكْبَرِ الْكَبَائِرِ … الإِشْرَاكُ بِاللَّهِ، وَعُقُوقُ الْوَالِدَيْنِ … أَلاَ وَقَوْلُ الزُّورِ وَشَهَادَةُ الزُّورِ
Narrated Abu Bakra:Allah's Messenger (ﷺ) said thrice, "Shall I not inform you of the biggest of the great sins?" We said, "Yes, O Allah's Messenger (ﷺ)" He said, "To join partners in worship with Allah: to be undutiful to one's parents." The Prophet (ﷺ) sat up after he had been reclining and added, "And I warn you against giving forged statement and a false witness; I warn you against giving a forged statement and a false witness." The Prophet kept on saying that warning till we thought that he would not stop.
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