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Under the Abbasids
Under the 8th-century Abbasids, Arabic zindiq and the adjectival zandaqa could "denote many different things, though it seems primarily (or at least initially) to have signified a follower of Manichaeism." However, many of those persecuted for zandaqa under the Abbasids claimed to be Muslims, and when applied to Muslims, the accusation was that the accused secretly harboured Manichaean beliefs. "The proof for such an accusation was sought, if at all, in an indication of some kind of dualism, or if that individual openly flouted Islamic beliefs or practices." As such, certain Muslim poets of early Abbasid times could thus also be accused of zandaqa as much as an actual Manichaean might.
The charge of zandaqa was a serious one, and could cost the accused his/her life. The "Slaughterer" caliph Abu al-'Abbas is cited as having said "tolerance is laudable, except in matters dangerous to religious belief, or to the sovereign's dignity." The third Abbasid caliph, Al-Mahdi, ordered the composition of polemnical works to refute freethinkers and other heretics, and for years he tried to exterminate them absolutely, hunting them down and exterminate freethinkers in large numbers, putting to death anyone on mere suspicion of being a zindiq. Al-Mahdi's successors, the caliphs al-Hadi and Harun al-Rashid, continued the pogroms, although with diminished intensity during the reign of the latter. Starting with al-Rashid's successor al-Ma'mun, religious persecution in Islam took a different direction with the institution of Mihna.
The reason for these persecutions are not easy to determine. Zandaqa was viewed as a threat to Islam, to Muslim society, and to the state. In the 8th century, Islamic norms were still under development and had not yet crystallized, and Muslims were still a small minority in the vast territories ruled by the caliphate, and even those who had converted were perceived to have been only "imperfectly" Islamized. Many of these converts had previously been Manichaeans, and Manichaeaism with its well developed missionary ideals had undergone a slight resurgence during early caliphate rule. As such, the Manichaeans were perceived as a threat to the security of the Muslim religious elite and to the Abbasid state. The threat was perceived to be especially evident in the quasi-scientific manner in which the Manichaeans posed unsettling questions, their skill at creating a favourable impression in public debate, and their ability in defending their own intellectually-appealing world-view.
In time, Muslim theologians came to apply zindiq to "the criminal dissident—the professing Muslim who holds beliefs or follows practices contrary to the central dogmas of Islam and is therefore to be regarded as an apostate and an infidel. The jurists differ as to the theoretical formulation of the point of exclusion, but in fact usually adopt the practical criterion of open rebellion."
In modern times, the term zindiq is occasionally used to denote members of religions, sects or cults that originated in a Muslim society but are considered heretical or independent faiths by mainstream Muslims.
Famous and alleged zindiqs
- Abu Nuwas
- Muhammad ibn Zakariya al-Razi:227–230
- Ibn al-Khatib
- Ibn al-Muqaffa'
- Abu Shakir
- Abu Tammar Muttabib 
- Abu Isa al-Warraq 
- Ibn al-Rawandi 
- Yazdan ibn Badhan (Persian: یزدان پور باذان)
- Bashar ibn Burd
- Abu 'All Raja' ibn Yazdanbakht, a Manichaean theologian during the caliphate of al-Ma'mun (813-833 CE).
- Abdulkarim ibn abi Al-Ouja'
- Khalid al-Qasri
- Lewis, Bernard (1993), Islam in history: ideas, people, and events in the Middle East, Open Court, p. 287.
- Zaman, Muhammad Qasim (1997), Religion and Politics Under the Early 'Abbasids: The Emrgence of the Proto-Sunni Elite, Brill, pp. 63–65.
- Bowker, John (1997), "Zindiq", The Concise Oxford Dictionary of World Religions, OUP.
- Glassé, Cyril (2013), "Zindiq", The New Encyclopedia of Islam (4th ed.), Rowman & Littlefield, p. 491.
- Hecht, Jennifer Michael (2003). Doubt: A History: The Great Doubters and Their Legacy of Innovation from Socrates and Jesus to Thomas Jefferson and Emily Dickinson. Harper San Francisco. ISBN 0-06-009795-7.
- Mirfetros, Ali (1978), "Zendiqs and Materialistic Thinkers", Hallaj (10th ed.), Alborz Press, pp. 102–126.
- Awasaf an-Nas fi Tawarikh wa Silat, pp19, Mohamed Kamal Chabana
- Zarrinkoub, Abdolhosein (1999), Two Centuries of Silence, ISBN 964-5983-33-6