Sin is a moral evil as considered from a religious standpoint. Sin is regarded in Judaism, Christianity and Islam as the deliberate and purposeful violation of the will of God. The concept of sin has been present in many cultures throughout history, where it was usually equated with an individual's failure to live up to external standards of conduct or with his violation of taboos, laws, or moral codes.
The breach of laws and norms laid down by Din Al-Islam, is considered as sin. The sin may be of omission or commission; though any violation of religious law or ethical norm is a sin, but one is held accountable only for those committed intentionally. Islam recognizes major and minor sins. Holding some one equal to God, or to share in His divinity is considered as the most serious unpardonable sin. Strict monotheism is the cardinal point of faith.
“If you avoid the great sins which you have been forbidden, We will do away with your small sins and cause you to enter a place of great honor (paradise).”(Qur’an;4:31, similarly at 53:32).
“Your Lord has decreed mercy upon Himself. If anyone among you commits evil because of ignorance and thereafter repents and mends his ways; you will find Allah Forgiving, Merciful."(Qur’an;6:54)
“.. Indeed virtues remove evils. This is a reminder for the mindful.”(Qur’an;11:114)
“Whatever affliction befalls you is the result of what your own hands have done even though for many of your misdeeds He grants forgiveness.”(Qur’an;42:30)
The Kharijites [Type of ‘Taliban’ of early 1st century Hijrah] were known for their puritanism and fanaticism. Any Muslim who committed a major sin was considered an apostate by them, this is rejected by renowned scholars.