nature of disbelief (kufr)

إِنَّ شَرَّ ٱلدَّوَآبِّ عِندَ ٱللَّهِ ٱلَّذِينَ كَفَرُوا۟ فَهُمْ لَا يُؤْمِنُونَ

Indeed, the worst of living creatures in the sight of Allāh are those who have disbelieved, and they will not [ever] believe-