3rd Ayah from Surah Al Munafiqun
ذَٰلِكَ بِأَنَّهُمۡ ءَامَنُواْ ثُمَّ كَفَرُواْ فَطُبِعَ عَلَىٰ قُلُوبِهِمۡ فَهُمۡ لَا يَفۡقَهُونَ ٣
Dhālika Bi'annahum 'Āmanū Thumma Kafarū Faţubi`a `Alá Qulūbihim Fahum Lā Yafqahūna
That is because they believed, and then they disbelieved; so their hearts were sealed over, and they do not understand.
because they professed faith and then rejected it, so their hearts have been sealed and they do not understand.
That is because they believed, then disbelieved, therefore their hearts are sealed, so they understand not.
That is because they declared faith (in Islam apparently), then disbelieved (secretly). Therefore a seal has been set on their hearts, and thus they do not understand.
That is because they believed, then disbelieved, therefore their hearts are sealed so that they understand not.
That is because they believed, then they rejected Faith: So a seal was set on their hearts: therefore they understand not.