Hardness of hearts due to rejection

وَنُنَزِّلُ مِنَ ٱلْقُرْءَانِ مَا هُوَ شِفَآءٌ وَرَحْمَةٌ لِّلْمُؤْمِنِينَ ۙ وَلَا يَزِيدُ ٱلظَّـٰلِمِينَ إِلَّا خَسَارًا

And We send down of the Qur’ān that which is healing and mercy for the believers, but it does not increase the wrongdoers except in loss.