Spiritual hardness of the heart (qaswat al-qalb)

وَلَقَدْ صَرَّفْنَا فِى هَـٰذَا ٱلْقُرْءَانِ لِيَذَّكَّرُوا۟ وَمَا يَزِيدُهُمْ إِلَّا نُفُورًا

And We have certainly diversified [the contents] in this Qur’ān that they [i.e., mankind] may be reminded, but it does not increase them [i.e., the disbelievers] except in aversion.