The contrast between external clarity of message and internal refusal to understand

وَقَالُوا۟ قُلُوبُنَا فِىٓ أَكِنَّةٍ مِّمَّا تَدْعُونَآ إِلَيْهِ وَفِىٓ ءَاذَانِنَا وَقْرٌ وَمِنۢ بَيْنِنَا وَبَيْنِكَ حِجَابٌ فَٱعْمَلْ إِنَّنَا عَـٰمِلُونَ

And they say, "Our hearts are within coverings [i.e., screened] from that to which you invite us, and in our ears is deafness, and between us and you is a partition, so work; indeed, we are working."