Divine knowledge of hidden hearts

وَمِنَ ٱلنَّاسِ مَن يَقُولُ ءَامَنَّا بِٱللَّهِ وَبِٱلْيَوْمِ ٱلْـَٔاخِرِ وَمَا هُم بِمُؤْمِنِينَ

And of the people are some who say, "We believe in Allāh and the Last Day," but they are not believers.