They doubt that God exists and therefore do not understand that a Supreme Being set moral standards for humans and that failure to uphold those standards amounts to sin.
وَفِي بَعْضِ ٱللُّغَاتِ ،‏ إِنْ قُلْتَ إِنَّ شَخْصًا مَا هُوَ خَاطِئٌ ،‏ فَقَدْ يَظُنُّ ٱلْآخَرُونَ أَنَّهُ ٱرْتَكَبَ جَرِيمَةً أَوْ عَلَى ٱلْأَقَلِّ خَالَفَ بَعْضَ ٱلْقَوَاعِدِ .‏