﴿ ذَٰلِكَ بِأَنَّهُمْ ءَامَنُوا۟ ثُمَّ كَفَرُوا۟ فَطُبِعَ عَلَىٰ قُلُوبِهِمْ فَهُمْ لَا يَفْقَهُونَ ﴾

“this, because [they profess that] they have attained to faith, whereas [inwardly] they deny the truth - and so, a seal has been set on their hearts so that they can no longer understand [what is true and what false].”

That namely their evil deed is because they believed by affirming faith only with the tongue then disbelieved in their hearts that is to say they persist in harbouring disbelief in it; therefore their hearts have been stamped sealed with disbelief. Hence they do not understand faith.