﴿ كَلَّا ۖ بَلْ ۜ رَانَ عَلَىٰ قُلُوبِهِم مَّا كَانُوا۟ يَكْسِبُونَ ﴾

“Nay, but their hearts are corroded by all [the evil] that they were wont to do!”

(Nay) truly, O Muhammad, (but that which they have earned) which they have done and said in their state of idolatry (is rust upon their hearts) Allah has sealed the hearts of those who disbelieve in the Day of Judgement; it is also said that the accumulation of sins blackens the heart and this is what is meant by the rust of hearts.