Hearts vs. outward actions

وَإِذَا جَآءُوكُمْ قَالُوٓا۟ ءَامَنَّا وَقَد دَّخَلُوا۟ بِٱلْكُفْرِ وَهُمْ قَدْ خَرَجُوا۟ بِهِۦ ۚ وَٱللَّهُ أَعْلَمُ بِمَا كَانُوا۟ يَكْتُمُونَ

And when they come to you, they say, "We believe." But they have entered with disbelief [in their hearts], and they have certainly left with it. And Allāh is most knowing of what they were concealing.