Surat Al Isra' (45-46)

{وَإِذَا قَرَأْتَ الْقُرآنَ جَعَلْنَا بَيْنَكَ وَبَيْنَ الَّذِينَ لاَ يُؤْمِنُونَ بِالآخِرَةِ حِجَاباً مَّسْتُوراً} (الإسراء: 45)

45- O Prophet, if you read the Qur'an, We make between you and the pagans who do not believe in Hereafter a barrier to prevent their hearts to understand and use. A group of them used to advise one another not to listen to the Qur'an, and when they heard it, they were keen not to be affected by, therefore Allah punished them.

{وَجَعَلْنَا عَلَى قُلُوبِهِمْ أَكِنَّةً أَن يَفْقَهُوهُ وَفِي آذَانِهِمْ وَقْراً وَإِذَا ذَكَرْتَ رَبَّكَ فِي الْقُرْآنِ وَحْدَهُ وَلَّوْاْ عَلَى أَدْبَارِهِمْ نُفُوراً} (الإسراء: 46)

46- We placed a cover over their hearts when they hear the Qur'an so as not to understand it, and would give them heavy ears so as not to benefit of hearing it. When you unify Allah during your recitation to the Qur'an, they turn their back as disinclination from the word of monotheism which means changing their polytheism and Pre-Islamic life.