Sealing of hearts (tabd'is)

وَجَعَلْنَا عَلَىٰ قُلُوبِهِمْ أَكِنَّةً أَن يَفْقَهُوهُ وَفِىٓ ءَاذَانِهِمْ وَقْرًا ۚ وَإِذَا ذَكَرْتَ رَبَّكَ فِى ٱلْقُرْءَانِ وَحْدَهُۥ وَلَّوْا۟ عَلَىٰٓ أَدْبَـٰرِهِمْ نُفُورًا

And We have placed over their hearts coverings, lest they understand it, and in their ears deafness. And when you mention your Lord alone in the Qur’ān, they turn back in aversion.