In The Name of Allaah, The Most Merciful, The Bestower of Mercy

Imaam Ibnul Qayyim [rahimahullaah] said: When sins become numerous, it has a severe effect on the heart of the [sinner], and thus the sinner deviates [from right conduct], just as some of the Salaf stated regarding the statement of Allaah  [The Most High]: [كَلَّا‌ۖ بَلۡۜ رَانَ عَلَىٰ قُلُوبِہِم مَّا كَانُواْ يَكۡسِبُونَ – Nay! But on their hearts is the Ran (covering of sins and evil deeds) which they used to earn]- Meaning: Sins in addition to other sins. Al-Hasan [rahimahullaah] said: It is sins in addition to other sins until the heart becomes blinded. And the basis of this is that the heart becomes decayed due disobedience, and when this disobedience increases, the decadence becomes more severe until the heart is covered in sins and evil deeds; then it becomes more severe until the heart is severely afflicted- locked and sealed [in sins and disobedience]. When this happens to the heart after it was in a state of guidance and clear-sightedness, it deteriorates severely and thus its noble state alternates to the lowest level of [misbehaviour, disobedience etc.]. Then its enemy [i.e. shaytaan] becomes its close ally and leads it wherever he wants.

[Source: Al-Jawaabul Kaafi’ page 139. Abridged and slightly paraphrased]