Sin darkens the heart. With every sin the light of the heart is destroyed and replaced with darkness. The door of mankind,s heart is open for Allah Almighty due the divine nature. This door is slightly closed with every single sin we commit. The human beings may exceed in committing sins and the door of his heart is closed thoroughly and the darkness takes over his heart and then no light would be able to permeate into his heart.
source : erfan.ir