The hardening of hearts (qaswat al-qalb) as a consequence of rejecting truth

لِّيَجْعَلَ مَا يُلْقِى ٱلشَّيْطَـٰنُ فِتْنَةً لِّلَّذِينَ فِى قُلُوبِهِم مَّرَضٌ وَٱلْقَاسِيَةِ قُلُوبُهُمْ ۗ وَإِنَّ ٱلظَّـٰلِمِينَ لَفِى شِقَاقٍۭ بَعِيدٍ

[That is] so He may make what Satan throws in [i.e., asserts] a trial for those within whose hearts is disease and those hard of heart. And indeed, the wrongdoers are in extreme dissension.