ذٰلِكَ بِأَنَّهُم آمَنوا ثُمَّ كَفَروا فَطُبِعَ عَلىٰ قُلوبِهِم فَهُم لا يَفقَهونَ

This is because they believed, then disbelieved, so their hearts were sealed and they don’t understand.