ذَلِكَ بِأَنَّهُمْ آمَنُوا ثُمَّ كَفَرُوا فَطُبِعَ عَلَى قُلُوبِهِمْ فَهُمْ لَا يَفْقَهُونَ
That is because they believed and then disbelieved, so their hearts were sealed. Hence, they do not understand.
Agha Ali Puya Commentary
Commentary on Quran 63:3
[Pooya/Ali Commentary 63:3] (see commentary for verse 1)