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Sin is an important concept in Islamic ethics. Muslims see sin as anything that goes against the commands of God (Allah), a breach of the laws and norms laid down by religion.Islam teaches that sin is an act and not a state of being. It is believed that God weighs an individual's good deeds and against his or her sins on the Day of Judgement and punishes those individuals whose evil deeds outweigh their good deeds. These individuals are thought to be sentenced to afterlife in the fires of ? jahannam (Hell).
The Quran describes these sins throughout the text and demonstrates that some sins are more punishable than others. A clear distinction is made between major and minor sins (53:31-32), indicating that if an individual stays away from the major sins, then he/she will be forgiven of the minor sins.
A number of different words for sin are used in the Islamic tradition.
According to A.J. Wensinck's entry in the Encyclopedia of Islam, Islamic terms for sin include dhanb and khaa, which are synonymous and refer to intentional sins; khi, which means simply a sin; and ithm, which is used for grave sins.
According to Cyril Glasse, Islam recognizes two kinds of sin (khati'ah): dhanb, a fault or shortcoming which is to be sanctioned; and ithm, a willful transgression which is to be punished.
Several different words are used in the Quran to describe sin--1) Dhanb 2) Ithm 3) Khati'ah 4) Jurm 5) Junah/Haraj. By examining the choice of words in Quranic verses used in connection with these terms, scholars have attempted to determine which sins are associated with which terms.
Dhanb (plural dhunub) is frequently applied to heinous sins committed against God. One of the main examples of Dhanb in the Quran is of "crying lies of God's signs", or having excessive pride that prevents an individual from believing the signs of God.
For in God's sight are (all) his servants, (namely), those who say: 'Our Lord, we have indeed believed: forgive us, then, our sins ("dhunub"), and save us from the agony of the Fire.'-- Quran 3: 15-16
This use of dhanb in the Quran exemplifies that this type of sin is punishable in the afterlife. In fact, dhanb is considered a 'great' sin and is often used in the Quran to contrast with sayyi'a, which denotes a 'smaller' sin. The Quran states that if you avoid these great sins, your lesser evil deeds or sayyi'at will be forgiven.
If you avoid great sins (kaba'ir or dhanb) which are forbidden you, We will remit from you your evil deeds (sayyi'a).-- Quran 4: 31 
Some scholars believe the basic meaning of ithm to be an unlawful deed that is committed intentionally. This contrasts to dhanb in that dhanb can be both intentional and unintentional. However, this definition is somewhat nebulous and the best description of the word is based on the contextual situations. In the Quran, ithm is found quite frequently in legislative descriptions. For example, falsely accusing your own wife in order to gain money is constituted as an ithm (Quran 4: 24/20). However, ithm is also used in connection with haram, or committing an unlawful deed, a taboo, such as consuming food or drink that is forbidden by God:
They will ask thee about wine and maysir. Say, 'In both of them there is great sin (ithm) and also some uses for men, but their sin is greater than their usefulness.'-- Quran 2: 168/173
He who associates with God has surely forged a great sin (ithm).-- Quran 4: 53/50 
This association with shirk is noteworthy for shirk is considered unforgivable if not repented of.
God forgiveth not (the sin of) joining other gods to Him; but He forgiveth whom He pleaseth other sins that this: one who joins other gods with God hath strayed far, far away.-- Quran 4:116
Khati'ah is considered by many scholars to be a "moral lapse" or a "mistake" [not specific enough to verify] This interpretation has led some scholars to believe that Khati'ah is a lesser sin than ithm; however, the word Khati'ah is frequently used in conjunction with ithm in the Quran.
Whoso, having committed a khati'ah or an ithm, throws it upon the innocent, has burdened himself with calumny and an obvious sin (ithm).-- Quran 4:112
This Quranic verse indicates that khati'ah is considered an ithm, a grave sin. In fact, the word khati'ah is associated with some of the most heinous religious sins in the Quran. In one Quranic verse this word is used to describe the sin of slaying one's own children for fear of poverty. (Quran 17:33/31). Scholars believe that dhanb or ithm could be used in place of khati'ah in this instance; however, the word choice indicates that khati'ah is more than just a moral lapse or mistake and is punishable.
The word Jurum is often considered to be a synonym of dhanb for it is used to describe some of the same sins: crying lies of God and not believing the signs of God. In the Quran, the word mostly appears in the form of mujrim, one who commits a jurm. These individuals are described in the Quran as having arrogance towards the believers.
Behold, those who commit jurm used to laugh at those who believed, winking one at another when they passed them by, and when they went back to their own fold, they returned jesting, and when they saw them they used to say, 'Lo, these have indeed gone astray!-- Quran 83: 29-32
Junah and Haraj have a similar meaning to that of ithm, a sin that warrants a punishment. In fact, these words are used almost interchangeably with ithm in the same chapters in the Quran. Like ithm, these words are found frequently in legislative portions of the Quran, particularly relating to regulations regarding marriage and divorce.
It is no sin (junah) for you that you offer proposal of marriage to women or keep it secret.-- Quran 2:235
The Prophet (Muhammad) said, "Piety is good manner, and sin is that which creates doubt and you do not like people to know it."
Wabisah bin Ma'bad reported:
"I went to Messenger of Allah (SAWS) and he asked me: "Have you come to inquire about piety?" I replied in the affirmative. Then he said: "Ask your heart regarding it. Piety is that which contents the soul and comforts the heart, and sin is that which causes doubts and perturbs the heart, even if people pronounce it lawful and give you verdicts on such matters again and again."-- Ahmad and Ad-Darmi
In Sunan al-Tirmidhi, a Hadith is narrated:
Allah's apostle said, "Every son of Adam sins, the best of the sinners are those who repent."-- Sunan al-Tirmidhi,Hadith no. 2499
Allah's apostle said," By Him in Whose Hand is my life, if you were not to commit sin, Allah would sweep you out of existence and He would replace (you by) those people who would commit sin and seek forgiveness from Allah, and He would have pardoned them."
According to Islam, one can be forgiven of sins through genuine tawbah (repentance) which literally means "to return."
Ask your Lord for forgiveness, then turn back to Him.-- Qur'an, sura 11, ayat 3
Unlike the Catholic concept of atonement, tawbah does not entail formal, ecclesiastical confession to a religious leader. Like Protestantism, Islam allows followers to repent directly to God. In addition, while Christianity and Islam considers humans as prone to sin, Islam ultimately views them as responsible for their actions and refutes the Christian concept of original sin.
For man's very soul incites him to evil unless my Lord shows mercy.-- Qur'an, sura 12, ayat 53
More so, in Islam Muslims are discouraged from confessing their sins and sharing the wrongdoings of others. 
Also, according to Islam, Blood sacrifice cannot add to Divine Grace nor replace the necessity of repentance. However, sacrifice is done to help the poor and to remember Abraham's willingness to sacrifice his son at God's command.
It is not their meat nor their blood that reaches Allah. it is your piety that reaches Him..."-- Qur'an, sura 22, ayat 37
When a human has violated another human's rights, ?uq?q al-?ib?d, compensation is necessary in order to show that one has made amends.
When a human has offended or disobeyed God, ?uq?q All?h, penitence, remorse, and resolution are necessary in order to show that one is sincere, and will not repeat the wrongdoing in the future.
According to Shaddad ibn Aws:
Shall I not how to seek forgiveness? O Allah, You are my Lord, there is no God but You; for You created me and I am Your servant; and I am upon Your covenant and Your promise as much as I am able; I seek refuge in You from the evil of what I have done; I acknowledge Your favors upon me and I recognize my sins, so forgive my sins; verily, none can forgive sins but You.-- Sunan At-Tirmidhi, Book of Supplications, Number 3393, Hasan
From a traditionalist perspective, sin is applied to an individual's actions. Through belief and good works, an individual can remove his/her sin and attain God's good favor. Classical legal scholar Muhammad al-Shafi'i (767 - 820) derived this understanding from Quranic passages such as:
But He will overlook the bad deeds of those who have faith, do good deeds, and believe in what has been sent down to Muhammad --the truth from their Lord --and He will put them into a good state.-- Qur'an, sura 47, ayat 2
From a modernist perspective, sin has also been applied to a group or community's collective behavior. Through public acknowledgement of wrongdoing, people can take responsibility for the lack of morality within their society and enact social reform. Egyptian reformer Mu?ammad ?Abduh (1849-1905) and his disciple Mu?ammad Rash?d Rid? (1865-1935) derived this understanding from Quranic passages such as:
Unless they repent, make amends, and declare the truth. I will certainly accept their repentance.-- Qur'an, sura 2, ayat 160
Other modern reformers, such as Sayyid Qutb, held that repentance involved a renewed, holistic commitment to Islam, rather than admission of sin for the sake of being pardoned of punishment. This understanding draws from classical Sufi thought, whereby one experiences a personality transformation and his/her sinful impulses are replaced by virtue. Qutb derived this understanding from Quranic passages such as:
Those who repent, believe, and do good deeds: God will change the evil deeds of such people into good ones.-- Qur'an, sura 25 ayat 70
Repentance for sin can be accomplished through acts such as, "fasting, giving charity, sacrificing an animal, and freeing a slave." In addition, going on the hajj can serve as a form of repentance.
According to Shaddad ibn Aws:
The Messenger of Allah, peace be upon him, said, "The lesser pilgrimage unto the lesser pilgrimage will expiate whatever sins were committed between them; and the accepted pilgrimage has no reward other than Paradise.-- Sahih Bukhari, Book 27, Number 1 
However, regardless of one's outward deeds, God does not accept the forgiveness of those who are insincere in their repentance and only do so in order to avoid jahannam.
It is not true repentance when people continue to do evil until death confronts them and then say, 'Now I repent.
The most heinous sins in Islam are known as Al-Kabirah (Persian: ? ) which translates to the great or major one. Some authors use the term enormity. While every sin is seen as an offense to Allah, the al-Kaba'ir are the gravest of the offenses. Allah's power is thought to be only eclipsed by his mercy and thus small sins are tacitly understood to be forgiven after repentance. Not every sin is equal however and some are thought to be more spiritually damning than others. The greatest of the sins described as al-Kaba'ir is the association of others with Allah or Shirk. Scholar differ as to how many major sins there are. In contrasting major sins with minor sins (al-sagha'ir), the eighth-century Shafi'i scholar Al-Dhahabi found the hadith collections of Sahih al-Bukhari and Muslim ibn al-Hajjaj listed seven major sins, while the tradition from Abd Allah ibn Abbas stated that there were closer to seventy major sins. 
Some of the major or al-Kaba'ir sins in Islam are as follows:
These are only the first sixteen in Islam and there are other fifty-four majors sins, and this list only opinion of particular scholars and do not wholly represent Islam.
Although many of the ideas for what is unacceptable overlap, the seven major sins of Islam differs from the seven deadly sins of Christianity. The Islamic sins refer more to specific undesirable behavior rather than to the general negative characteristics or actions of the cardinal Christian sins. Despite the similar names, the seven main sins in Islam are more comparable to the Ten Commandments rather than the seven deadly sins. They both provide the bottom line for believers in terms of what is acceptable behavior in the faith. The actions themselves differ most of the major crimes in Islam relate to subservience to Allah. Any form of polytheism is seen to be the most severe offense in the religion and all of the other transgressions are in some form of association with Allah. Witchcraft, for example, is the taking on of supernatural powers in order to make the practitioner a being above the normal human. This challenges the power of Allah as the person in question has superseded their mortal position to become something greater and akin to a god. The same can be said of murder, as ultimately the power to decide who shall live and die is believed to belong solely to Allah. Life is thought to be a gift from Allah and the unjust taking of life is a severe spiritual offense, as it is not only seen as morally wrong but also as an affront to God.
In addition to what Muslim scholars agree are the principal seven sins, the idea exists that the major sins extend far past the seven. These additional transgressions, potentially up to seventy, are not universally settled upon nor are they explicitly stated in the Qur'an, however they are thought to be implied by the text. The supplementary sins as a whole lack the spiritual gravity of the original seven and include things such as drinking alcohol and eavesdropping.
In Islam sin is divided into two categories. The first is that of dhanb, which is a fault or shortcoming, a limitation, an inadvertencies, the consequence of which is a sanction rather than a punishment. Sin as dhanb is distinguished from willful transgression (ithm), which is more serious and clearly incurs punishment rather than sanction. ... The term khati'ah is used in practice indiscriminately for both concepts of sin.