Shirk (associating partners with Allah)الشِّرك بالله
إِنَّ اللَّهَ لَا يَغْفِرُ أَن يُشْرَكَ بِهِ وَيَغْفِرُ مَا دُونَ ذَٰلِكَ لِمَن يَشَاءُ ۚ وَمَن يُشْرِكْ بِاللَّهِ فَقَدِ افْتَرَىٰ إِثْمًا عَظِيمًا
"Indeed, Allah does not forgive associating partners with Him, but He forgives what is less than that for whom He wills. And whoever associates others with Allah has certainly fabricated a tremendous sin."
Shirk is the one sin Allah will not forgive if a person dies upon it, because it denies the very purpose of creation: to worship Him alone. It nullifies all good deeds.الشرك هو الذنب الوحيد الذي لا يغفره الله لمن مات عليه، لأنه يناقض الغاية من الخلق: أن يُعبد وحده. وهو يُحبط كل عملٍ صالح.