Corruption of belief (kufr and doubt)

وَذَٰلِكُمْ ظَنُّكُمُ ٱلَّذِى ظَنَنتُم بِرَبِّكُمْ أَرْدَىٰكُمْ فَأَصْبَحْتُم مِّنَ ٱلْخَـٰسِرِينَ

And that was your assumption which you assumed about your Lord. It has brought you to ruin, and you have become among the losers."