Quran

 63:3

ذَٰلِكَ بِأَنَّهُمْ آمَنُوا ثُمَّ كَفَرُوا فَطُبِعَ عَلَىٰ قُلُوبِهِمْ فَهُمْ لَا يَفْقَهُونَ ﴿٣﴾

3. That is because they believed and then disbelieved. So a seal was set upon their hearts, so they do not understand.