Since the earliest beginnings of Shiism are located approximately within the same milieu in which Islam emerged, a lot of what can be said about influences on Islam generally speaking also applies to its Shiite branch. It was a milieu shaped by late classical political and religious culture with its superpowers, the Christian Byzantine and the Zoroastrian Sassanian Empires. Judaism and Christianity—and thus monotheism—were well entrenched in the area where Islam emerged, though polytheism was also common.
Muhammad successfully combined the figure of the emperor with that of the holy man, another stock character of late antiquity. He presented himself as a prophet, and perhaps the last one, in the line of the biblical prophets who brought the truth in its pure form after previous revelations had been distorted. Yet, Islam was also an Arabian religion that had integrated the pre-Islamic heritage of the Bedouins and southern Arabians. The combination of several sources of charismatic authority—learning, genealogy, and unusual claims—probably appealed to different communities in the late classical Middle East. While these legacies shaped all of Islam, some help to contextualize distinctly Shiite features.
Like all of Islam, Shiism is part of the monotheistic, biblical tradition with its one God and its prophets. Shiism reflects this ancestry when prophets before Muhammad are interpreted according to the same paradigm as the Imams and Husayn's martyrdom is compared to Jesus' sacrifice. The parallels between Twelver Shiite and Jewish dietary law may suggest a direct influence. Another legacy from the ancient world is the importance assigned in esoteric systems to numbers such as twelve or seven. Other traditions also had an impact on trends that existed in both Shiite and Sunni Islam. While the influence of Greek philosophy on the Muslim falasifa may be a general phenomenon, its Neoplatonic elements gained a particular importance for Shiite thought in the Isfahan school of theosophers.
The main center of Shiism in the earliest days was Kufa in Iraq, founded in 638 as a garrison town. The influence of Kufan local politics on Shiism was mostly indirect, although this remains controversial among modern scholars. The composition of the city's population and the ambitions of its leading elites regarding their place in the Islamic world determined to a certain degree the circles among which Shiism became popular, and these in turn contributed to its future shape. Thus, even though the new Islamic identity would eventually supersede the priority of old tribal affiliations, the latter were very much alive during the Umayyad period (661-750).
The traditional tribal leaders of the Arab Muslim soldiers first supported Ali, in whom they had seen a natural ally against Uthman, but later abandoned his cause. The support for the Shiites in Kufa was mostly popular and well established among the large numbers of those Muslim soldiers deployed in the garrison who were of south Arabian origin. According to some historians, their traditions of political leadership may have predisposed them toward the authority of the Alids (the dynasty descended from Ali), since in both cases charismatic leadership is based on genealogical distinction. Another community in Kufa was that of the mawali, non-Arab converts to Islam who worked in small businesses while the Arabs constituted the army. This milieu produced some of the more extreme features of Shiism. The connection between Shiism and Kufa had further consequences. Kufa and Basra were the two administrative centers during the Islamic expansion into Iran. Areas assigned to Kufa (Qom, Azerbaijan) would inherit the city's Shiite tendency.
Because of the early popularity of Shiism among non-Arab converts to Islam, it is not surprising that ideas which circulated in the pre-Islamic Near East influenced this branch of Islam, especially its marginal forms. The heterodox beliefs of 'extreme' Shiites (Arabic: ghulat) are often attributed to Gnostic ideas. This religious trend, which existed among ancient and late classical Jews and Christians, posits a radical dichotomy between the evil created world and the divine spiritual world. Humans are specks of divine light caught in the material sphere and need to acquire the secret knowledge of gnosis in order to be saved and return to their home sphere. This knowledge involves dismissing the laws. This correlates to the antinomianism of some Shiite sects. The Gnostic legacy can also be seen in the general assumption that the Imams have knowledge that grants salvation and—again among more extreme groups—in the deification of the Imams and the belief in transmigration of souls.