Imam Muhammad al-Baqir (as): Every single person has a white spot on his heart, and when he commits a sin, a black dot appears in the white spot, so if he repents, the black dot disappears, but if he continues to indulge in sins, the black dot expands until it engulfs the white spot and when that happens, he will never return to good and that is the purport of God's verse in the Quran: "...No, that is not the case! Rather, their hearts have been sullied by what they have been earning." al-Kāfī