Al-Munafiqun - 63:3

ذَلِكَ بِأَنَّهُمْ آمَنُوا ثُمَّ كَفَرُوا فَطُبِعَ عَلَى قُلُوبِهِمْ فَهُمْ لَا يَفْقَهُونَ
[63:3]
That is because they believe, then disbelieve, so a seal is set upon their hearts so that they do not understand.
That is because they believed, then they rejected Faith: So a seal was set on their hearts: therefore they understand not.
That is because they believed, then disbelieved, therefore their hearts are sealed so that they understand not.