People of the Book
The status of the Ahl al-Kitab (People of the Book), particularly Jews and Christians, with respect to the Islamic notions of unbelief is not clearcut. Charles Adams writes that the Quran reproaches the People of the Book with kufr for rejecting Muhammad's message when they should have been the first to accept it as possessors of earlier revelations, and singles out Christians for disregarding the evidence of God's unity. The Quranic verse 5:73 ("Certainly they disbelieve [kafara] who say: God is the third of three"), among other verses, has been traditionally understood in Islam as rejection of the Christian Trinity doctrine, though modern scholarship has suggested alternative interpretations. Other Quranic verses strongly deny the deity of Jesus Christ, son of Mary and reproach the people who treat Jesus as equal with God as disbelievers who will have strayed from the path of God which would result in the entrance of hellfire. While the Quran does not recognize the attribute of Jesus as the Son of God or God himself, it respects Jesus as a prophet and messenger of God sent to children of Israel. Some Muslim thinkers such as Mohamed Talbi have viewed the most extreme Quranic presentations of the dogmas of the Trinity and divinity of Jesus (5:19, 5:75-76, 5:119) as non-Christian formulas that were rejected by the Church.[32]Cyril Glasse criticizes the use of kafirun [pl. of kafir] to describe Christians as "loose usage". According to the Encyclopedia of Islam, in traditional Islamic jurisprudence, ahl al-kitab are "usually regarded more leniently than other kuffar [pl. of kafir]..." and "in theory" a Muslim commits a punishable offense if he says to a Jew or a Christian: "Thou unbeliever".
Historically, People of the Book permanently residing under Islamic rule were entitled to a special status known as dhimmi, while those visiting Muslim lands received a different status known as musta'min.
Mushrikun
Mushrikun (pl. of mushrik) are those who practice shirk, which literally means "association" and refers to accepting other gods and divinities alongside the god of the Muslims - Allah (as God's "associates"). The term is often translated as polytheism. The Quran distinguishes between mushrikun and People of the Book, reserving the former term for idol worshipers, although some classical commentators considered Christian doctrine to be a form of shirk. Shirk is held to be the worst form of disbelief, and it is identified in the Quran as the only sin that God will not pardon (4:48, 4:116).Accusations of shirk have been common in religious polemics within Islam. Thus, in the early Islamic debates on free will and theodicy, Sunni theologians charged their Mu'tazila adversaries with shirk, accusing them of attributing to man creative powers comparable to those of God in both originating and executing his own actions. Mu'tazila theologians, in turn, charged the Sunnis with shirk on the grounds that under their doctrine a voluntary human act would result from an "association" between God, who creates the act, and the individual who appropriates it by carrying it out.
In classical jurisprudence, Islamic religious tolerance applied only to the People of the Book, while mushrikun, based on the Sword Verse, faced a choice between conversion to Islam and fight to the death, which may be substituted by enslavement. In practice, the designation of People of the Book and the dhimmi status was extended even to non-monotheistic religions of conquered peoples, such as Hinduism. Following destruction of major Hindu temples during the Muslim conquests in South Asia, Hindus and Muslims on the subcontinent came to share a number of popular religious practices and beliefs, such as veneration of Sufi saints and worship at Sufi dargahs, although Hindus may worship at Hindu shrines also.
Sinners
Whether a Muslim could commit a sin great enough to become a kafir was disputed by jurists in the early centuries of Islam. The most tolerant view (that of the Murji'ah) was that even those who had committed a major sin (kabira) were still believers and "their fate was left to God". The most strict view (that of Kharidji Ibadis, descended from the Kharijites) was that every Muslim who dies having not repented of his sins was considered a kafir. In between these two positions, the Mu'tazila believed that there was a status between believer and unbeliever called "rejected" or fasiq.Takfir
The Kharijites view that the self-proclaimed Muslim who had sinned and "failed to repent had ipso facto excluded himself from the community, and was hence a kafir" (a practice known as takfir) was considered so extreme by the Sunni majority that they in turn declared the Kharijites kafir, following the hadith that declared, "If a Muslim charges a fellow Muslim with kufr, he is himself a kafir if the accusation should prove untrue".Nevertheless, in Islamic theological polemics kafir was "a frequent term for the Muslim protagonist" holding the opposite view, according to Brill's Islamic Encyclopedia.
Present day Muslims who make interpretations that differ from what others believe are declared kafirs, fatwas (edicts by Islamic religious leaders) are issued ordering Muslims to kill them and some such people have been killed also.
