Tales Of The Old 83:13
That is, they have no good reason for regarding the meting out of rewards and punishments as unreal. What has made them say this, is that their hearts have become rusty because of their sins; therefore, a thing which is thoroughly reasonable appears unreal and unreasonable to them. The explanation of this rust as given by the Holy Prophet (upon whom be peace) is as follows: “When a servant commits a sin, it marks a black stain on his heart. If he offers repentance, the stain is washed off, but if he persists in wrongdoing. it spreads over the whole heart.” (Musnad Ahmad, Tirmidhi, Nasa’i, Ibn Majah, Ibn Jarir, Hakim, Ibn Abi Hatim, Ibn Hibban and others). Reference: Tafheem ul Qur’an