Indeed, all sins poison the heart and lead to its illness and destruction. They sicken it and define its will as other than what Allah, the Almighty wants [of His servant], in effect being as harmful to the heart as actual poisons are to the body. Imam Ibn Al-Mubarak said, “I have seen that sins cause the death of the heart and addiction to them breeds humiliation. On the other hand, quitting sins gives life to the heart and it is better for one to disobey his own self [than to disobey Allah, the Almighty].”

The extent of harm that sins cause to the heart and body in this life and in the Hereafter, are known to Allah, the Almighty alone. However, every evil and every single disease in this life and the Hereafter, can be attributed to sins and acts of disobedience. Ibn Al-Qayyim said:

“Then why [else] were our parents (Aadam and Eve, may Allah exalt their mention) expelled from Paradise – the abode of joy, pleasure, bliss and happiness – to the dwelling of suffering, sorrows and disasters? Why was Satan cast out from the kingdom of Heaven? Why was he cursed and deformed externally and internally, until he had the most heinous appearance, with his insides even uglier and more abominable than his exterior? Why was he under a curse instead of mercy, and given ugliness in place of beauty, and Hell instead of Paradise? He became worthless in the sight of Allah, the Almighty, deprived of His compassion and inflicted with His anger. Allah, the Almighty deluded and detested him so much that He led him to destruction, until he became a leader for every immoral and criminal person; and Satan accepted this position although he had [earlier] worshipped [Allah]. We seek refuge with You, O Allah from breaching Your command or doing what You prohibited!”

He continued:

“Why did all the people on earth drown [at the time of Noah, may Allah exalt his mention] and the water submerge the peaks of mountains? Why were the people of ‘Aad afflicted with an intense wind that left their lifeless bodies on earth like they were hollow trunks of palm trees? Why was the tribe of Thamood afflicted with a loud shriek that literally tore their hearts on the inside and all of them died [as a result]? Why were the villages of the people of Louth lifted so high up to the sky that the angels heard the barking of their dogs, and then were turned upside down, leading to their deaths; and why were they cast with stones of layered, hard clay? In this way, they were punished twice, which was something that did not happen to any other nation. Their counterparts today may also have the same punishment, and this is never far from the wrongdoers. As for the people of Shu‘ayb why were they afflicted with clouds of torment that appeared as canopies, but when they approached over their heads, they rained flames of blazing fire over them? Why were Pharaoh and his soldiers drowned in the sea and transported by soul to Hell? Why was Qaroon, along with his house, property and family, swallowed by the earth? Why were the generations after Nooh (Noah) destroyed by different kinds of punishment? What about the Children of Israel; why were they tried by militarily superior people, who ransacked their homes, killing men and taking children and women captive, in the process? Why did those enemies later again come to them and destroy everything they could to the point of annihilation? Why were they afflicted with different types of torturous punishments: death, imprisonment, destruction of their houses, subjugation to the tyranny of kings and transformation into apes and pigs? The last of these hardships is mentioned in the verse where Allah, the Almighty Says (what means): {He Would surely [continue to] send upon them until the Day of Resurrection those who would afflict them with the worst torment} [Quran 7:167]”

Some of the effects of acts of disobedience and sins:

• They are ammunition that a sinner provides his eneymy with, supporting his rival with an army against his very own self.
• They They embolden others to defy the sinner, in ways they would never have previously dared.
• They gradually seal one’s heart; if sins accumulate, their doer becomes heedless. Some of the righteous predecessors said that the verse where Allah, the Almighty Said (what means): {No! Rather, the stains has covered their hearts of that which they were earning} [Quran 83:14], refers to the sins that pile up until the heart is blinded. That is because the heart rusts from sins, an increase of which steps up corrosion until it constitutes rust. This, then, in turn, develops until it becomes a seal and bolt which enlarges, until it completely wraps around the heart.
• The enormity of sins is removed from the heart to the extent that it then considers them as mere habits.
• Sins breed sins and help sustain the [habit of committing] evil.
• Evil deeds render darkness in one’s heart, which is as tangible as the shadows of the night. It was narrated on the authority of Ibn ‘Abbas that he said, “A good deed brings about light in [its doer’s] face and heart, blessing in provisions and love in the hearts of people. Conversely, a sin brings about darkness in the face and the heart, lack in sustenance and hatred in the hearts of people.”
• Sins weaken the heart and body. As for the former, it is evident in its continuous pattern which ends only when its life does. The body, too, becomes weakened because a believer’s power lies in his heart, and the stronger that is, the stronger his body is.
• They render man’s affairs difficult; whenever he wants to do something, he finds it tough or unattainable. One of the righteous predecessors said, “When I disobey Allah, the Almighty, I find that reflected in the behavior of my wife and [even my] domesticated animal.”
• Sins create alienation between its doer and others, especially the good among them. Abu Ad-Dardaa’ advised, “Beware of being cursed by the hearts of the believers while you do not know! You secretly commit sins and that is why Allah, the Almighty casts hatred of you in their hearts.”
• Sins dismiss a man’s honor and dignity in the sight of Allah, the Almighty and His creatures, as He Says (what means): {And he whom Allah humiliates - for him there is no bestower of honor} [Quran 22:18]
• Evil deeds not only extinguish the fire of zeal in one’s heart, but also removes bashfulness from it, which is the source of life to the heart.
• Sins diminish the heart’s zeal to please Allah, the Almighty and win in the Hereafter.
• A person continues to commit vices until he or she belittles them and considers them as trivial. Ibn Mas‘ood said: “The believing man sees his sin as if it were a mountain that he fears will fall down on him; whereas the immoral person thinks of his sin as a fly that sat on his nose and he easily waved it off.”[Imam At-Tirmithi] Also, Anas, may Allah be pleased with him, once remarked: “Indeed, you commit actions that you consider as trivial, but we used to consider them at the time of the Prophet to be major sins.” [Imam Al-Bukhari]

Bilal Ibn Sa‘ad advised, “Do not look at the smallness of a sin but look at the magnificence of the One Whom you disobeyed.”

As for the foremost five evil acts that afflict the heart and are most widespread, they are excess of: talking, prying gaze, socializing, eating and sleeping.