ذَٰلِكَ بِأَنَّهُمۡ ءَامَنُواْ ثُمَّ كَفَرُواْ فَطُبِعَ عَلَىٰ قُلُوبِهِمۡ فَهُمۡ لَا يَفۡقَهُونَ ٣
That is because they believed, and then they disbelieved; so their hearts were sealed over, and they do not understand.
Translation: Saheeh International (English)
Each word's meaning and sound
That is because they hypocritically professed faith, but faith did not reach their hearts, and then they secretly rejected Allah, so He put a seal on their hearts on account of their disbelief so that faith cannot enter it. Due to this seal they do not understand that in which lies their rectitude and righteousness.
Al-Mukhtasar in interpreting the Noble Quran