First, Allah the Almighty said: “Thus when they swerved [from the right path] God caused their hearts to swerve [from right guidance}.” (As-Saff 61:5)

 

In other words, Allah processed that which people chose for themselves. The same thing is for believers seeking guidance, Allah the Almighty said: “Those who strive for Us, We shall most surely guide them upon Our pathways [to salvation].” (Al-`Ankabut 29:69)

 

Second, Allah the Almighty spoke of these people’s nature that even when they are resurrected and were able to see the whole truth if they were send back to live again they go back to their nature of rejecting the truth. “If they were to be returned [to the world] they would revert to all [the ways] that have been forbidden].”  (Al-An`am 6:28)

 

So, God the All-Knowing knew their nature and what they chose for themselves. So, He processed it to them that the hearts were in some sort of a seal or stamp.

 

Almighty Allah knows best.