The pre-Islamic and non-Islamic legacy with which early Islamic theology came into contact included almost all the religious thought that had survived and was being defended or disputed in Egypt, Syria, Iran, and India. It was transmitted by learned representatives of various Christian, Jewish, Manichaean (members of a dualistic religion founded by Mani, an Iranian prophet, in the 3rd century), Zoroastrian (members of a monotheistic, but later dualistic, religion founded by Zoroaster, an Iranian prophet who lived before the 6th century bce), Indian (Hindu and Buddhist, primarily), and Ṣābian (star worshippers of Harran often confused with the Mandaeans) communities and by early converts to Islam conversant with the teachings, sacred writings, and doctrinal history of the religions of these areas. At first, access to this legacy was primarily through conversations and disputations with such men, rather than through full and accurate translations of sacred texts or theological and philosophic writings, although some translations from Pahlavi (a Middle Persian dialect), Syriac, and Greek must also have been available.
The characteristic approach of early Islamic theology to non-Muslim literature was through oral disputations, the starting points of which were the statements presented or defended (orally) by the opponents. Oral disputation continued to be used in theology for centuries, and most theological writings reproduce or imitate that form. From such oral and written disputations, writers on religions and sects collected much of their information about non-Muslim sects. Much of Hellenistic (post-3rd-century-bce Greek cultural), Iranian, and Indian religious thought was thus encountered in an informal and indirect manner.
From the 9th century onward, theologians had access to an increasingly larger body of translated texts, but by then they had taken most of their basic positions. They made a selective use of the translation literature, ignoring most of what was not useful to them until the mystical theologian al-Ghazālī (flourished 11th–12th centuries) showed them the way to study it, distinguish between the harmless and harmful doctrines contained in it, and refute the latter. By this time Islamic theology had coined a vast number of technical terms, and theologians (e.g., al-Jāḥiẓ) had forged Arabic into a versatile language of science; Arabic philology had matured; and the religious sciences (jurisprudence, the study of the Qurʾān, Hadith, criticism, and history) had developed complex techniques of textual study and interpretation. The 9th-century translators availed themselves of these advances to meet the needs of patrons. Apart from demands for medical and mathematical works, the translation of Greek learning was fostered by the early ʿAbbāsid caliphs (8th–9th centuries) and their viziers as additional weapons (the primary weapon was theology itself) against the threat of Manichaeism and other subversive ideas that went under the name zandaqah (“heresy” or “atheism”). Muhsin S. Mahdi The Editors of Encyclopaedia Britannica Theology and sectarianism
Despite the notion of a unified and consolidated community, as taught by the Prophet Muhammad, serious differences arose within the Muslim community immediately after his death. According to the Sunnis—the traditionalist faction whose followers now constitute the majority branch of Islam—the Prophet had designated no successor. Thus, the Muslims at Medina decided to elect a chief. Two of Muhammad’s fathers-in-law, who were highly respected early converts as well as trusted lieutenants, prevailed upon the Medinans to elect a leader who would be accepted by the Quraysh, Muhammad’s tribe, and the choice fell upon Abū Bakr, father of the Prophet’s favoured wife, ʿĀʾishah. All of this occurred before the Prophet’s burial (under the floor of ʿĀʾishah’s hut, alongside the courtyard of the mosque).
According to the Shiʿah, however, the Prophet had designated as his successor his son-in-law ʿAlī ibn Abī Ṭālib, husband of his daughter Fāṭimah and father of his only surviving grandsons, Ḥasan and Ḥusayn. His preference was general knowledge. Yet, while ʿAlī and the Prophet’s closest kinsmen were preparing the body for burial, Abū Bakr, ʿUmar, and Abū ʿUbaydah, from Muhammad’s companions in the Quraysh tribe, met with the leaders of the Medinans and agreed to elect the aging Abū Bakr as the successor (khalīfah, hence “caliph”) of the Prophet. ʿAlī and his kinsmen were dismayed but agreed for the sake of unity to accept the fait accompli because ʿAlī was still young.
After the murder of ʿUthmān, the third caliph, ʿAlī was invited by the Muslims at Medina to accept the caliphate. Thus, ʿAli became the fourth caliph (656–661), but the disagreement over his right of succession brought about a major schism in Islam, between the Shiʿah—those loyal to ʿAlī—and the Sunni “traditionalists.” Athough their differences were in the first instance political, arising out of the question of leadership, theological differences developed over time. The Khārijites
During the reign of the third caliph, ʿUthmān, certain rebellious groups accused the caliph of nepotism and misrule, and the resulting discontent led to his assassination. The rebels then recognized the Prophet’s cousin and son-in-law, ʿAlī, as ruler but later deserted him and fought against him, accusing him of having committed a grave sin in submitting his claim to the caliphate to arbitration. The name Khārijite (khārijī) is derived from the word khārajū, meaning “to withdraw,” because the Khārijites withdrew (by active dissent or rebellion) from a state of affairs they considered to be gravely impious.
The basic doctrine of the Khārijites was that a person or a group who committed a grave error or sin and did not sincerely repent ceased to be Muslim. Mere profession of the faith—“there is no god but God; Muhammad is the prophet of God”—did not make a person a Muslim unless this faith was accompanied by righteous deeds. In other words, good works were an integral part of faith and not extraneous to it. The second principle that flowed from their aggressive idealism was militancy, or jihad, which the Khārijites considered to be among the cardinal principles, or pillars, of Islam. Contrary to the orthodox view, they interpreted the Qurʾānic command about “enjoining good and forbidding evil” to mean the vindication of truth through the sword. The placing of these two principles together made the Khārijites highly inflammable fanatics, intolerant of almost any established political authority. They incessantly resorted to rebellion and, as a result, were virtually wiped out during the first two centuries of Islam.
Because the Khārijites believed that the basis of rule was righteous character and piety alone, any Muslim, irrespective of race, colour, and sex, could, in their view, become ruler—provided he or she satisfied the conditions of piety. This was in contrast to the claims of the Shiʿah (the party of Muhammad’s son-in-law, ʿAlī) that the ruler must belong to the family of the Prophet and in contrast to the doctrine of the Sunnis (followers of the Prophet’s way) that the head of state must belong to the Prophet’s tribe, the Quraysh.