And We turn their hearts... And We leave them is an addition to 'they believe,' included in the ruling of 'what makes you know,' meaning: 'What makes you know that they do not believe, and what makes you know that We turn their hearts and their sights?' That is, We seal their hearts and their sights so they do not understand or see the truth as they did when Our verses were revealed. Or they do not believe in it because their hearts are sealed. And what makes you know that We leave them in their transgression? That is, We let them be and their affair, We do not stop them from transgression until they are engulfed in it. [The saying 'until they are engulfed in it' means they become confused.] And it has been read: 'and He turns.' And 'and He leaves them' with the pronoun referring to Allah, the Exalted and Majestic. Al-A'mash read: 'and their hearts and their sights are turned,' in the passive form.