Islam's largest internal divide did not begin with a doctrine but with an unresolved question: after Muhammad's death, who was entitled to lead the community of believers? A counter-intuitive point: Sunnis and Shias share the same Qur'an, the same Prophet, the same testimony of faith, the same Five Pillars. What first separated them was a dispute over succession; only over the following centuries did it settle into distinct theology, law, and collective memory.
"This day I have perfected for you your religion."—— Qur'an 5:3, traditionally placed after Muhammad's farewell pilgrimage in 632; the question of "who comes next" is precisely where Sunni and Shia part ways
When Muhammad died in 632 CE, he left no undisputed arrangement for a successor and no surviving son. Who would lead the ummah (community of believers)? Two answers emerged, later crystallizing into Sunni and Shia.
The majority held that leadership should fall to a worthy figure chosen by the community's consensus. Thus the Prophet's close companion Abu Bakr was acclaimed as the first caliph (khalifa, "successor"). Umar, Uthman, and Ali followed, later revered by Sunnis as the "Rightly Guided Caliphs" (Rashidun). The name Sunni derives from ahl al-sunna—"the people of the Prophet's practice and the community's consensus."
Another group believed the Prophet had designated his cousin and son-in-law Ali ibn Abi Talib as his heir (they place special weight on a declaration at Ghadir Khumm during the farewell journey), and that leadership should remain within the Prophet's family (ahl al-bayt, the household). Ali's partisans were called shi'at Ali—"the party of Ali," the origin of the name Shia. Ali did eventually become the fourth caliph, but was assassinated in 661 CE.
What forged the political rift into an irreversible religious identity was the Battle of Karbala in 680 CE. The Umayyad dynasty had made the caliphate hereditary; Ali's son and the Prophet's grandson Husayn refused to recognize its legitimacy and, with a few dozen followers, was surrounded and killed by an army of thousands at Karbala in present-day Iraq. Husayn's martyrdom became, for Shias, the supreme witness against unjust power, mourned each year on Ashura (the tenth day of Muharram in the Islamic calendar). Over the following centuries, both traditions developed mature systems of law and theology.
Among the world's Muslims, Sunnis make up roughly 85%–90% and Shias roughly 10%–13% (somewhere between 150 and 240 million, with ranges depending on the definition used). Sunnis are the mainstream in nearly every Muslim society from West Africa to Southeast Asia; Shias are more geographically concentrated:
A point worth making plainly: Shia does not equal Persian, and Sunni does not equal Arab. Iran may be the Shia heartland, but there are many Shias among Arabs and South Asians alike; meanwhile the overwhelming majority of Muslims across the Arab world, South Asia, and Southeast Asia are Sunni. The two are geographically interwoven and have historically lived side by side far more often than in separated blocs—the antagonism seen in some regions in recent decades often overlays geopolitics and state rivalry rather than pure doctrinal conflict.
Both traditions affirm the oneness of God (tawhid), the Qur'an, Muhammad as the seal of the prophets, and resurrection and judgment—the skeleton of Islamic belief is shared. The real divide turns on one question: after the Prophet, who carries religious authority, and how is it transmitted?
Sunni Islam has, in principle, no clerical hierarchy; the caliph is a political and communal leader, not an infallible religious intermediary. Religious authority derives from learning (the ulama, the scholars) and consensus (ijma), embodied in four schools of law—Hanafi, Maliki, Shafi'i, and Hanbali—which recognize one another and hold sway in different regions. The believer stands directly before God, without priestly mediation.
Shias believe that religious and spiritual leadership after the Prophet belongs to a divinely chosen Imam drawn from the household—not merely a political leader but the authoritative interpreter of scripture, divinely protected from error. The mainstream Twelvers hold that there were twelve Imams, the last of whom entered "occultation" (ghayba) in the ninth century and will return at the end of time as the Mahdi to establish justice. Other Shia branches include the Ismailis and the Zaydis. Their school of law is the Ja'fari school (named for the sixth Imam, Ja'far al-Sadiq).
In everyday devotion, both traditions keep the testimony of faith, prayer, almsgiving, fasting, and pilgrimage—a Sunni and a Shia circling the Kaaba in Mecca do so with much the same motions. The differences lie in details, legal sources, and collective memory:
Yet it bears emphasizing: these differences sit atop a vast shared foundation. Across most periods and places, Sunnis and Shias have lived as neighbors, intermarried, and shared markets. To imagine them as two implacably opposed religions badly oversimplifies the internal picture of Islam.
Misreading: Sunni and Shia are two different religions.
Both belong to Islam and share the Qur'an, the Prophet, the testimony of faith, and the Five Pillars. The divide arose over leadership after the Prophet—a difference of tradition within Islam, not a difference of religions.
Misreading: Shias "worship" Ali or the Imams.
Shias hold strictly to the oneness of God and worship God alone. They revere, commemorate, and visit the shrines of the household and the Imams as exemplars and intercessory objects of love—which is fundamentally different from worship, just as revering the Prophet is not worshipping him.
Misreading: Sunnis and Shias have been at war since the beginning.
Historically the two have mostly lived side by side and intermarried. The antagonism seen in some regions today usually overlays state rivalry, geopolitics, and identity mobilization; reducing it to a fixed 1,400-year "sectarian blood feud" is inaccurate.
Misreading: Shia means Iranian/Persian.
Shias are found across the Arab world (Iraq, Lebanon, the Gulf) and South Asia, and are mostly of Arab or South Asian ethnicity; Iran is simply the single country where they are most numerous and influential. Ethnicity and sect do not map onto each other.
A universal problem of "succession." After a founder's death, how is authority continued? This is not a dilemma unique to Islam. Christianity, after the apostles, appealed to "apostolic succession" and the episcopate (Days 3 and 4); Judaism, after the Temple, turned to rabbis and the oral law (Day 1). Sunnis hand the answer to learning and consensus, Shias to a designated line—a contrast worth setting beside the Catholic/Orthodox "succession" and the Protestant "scripture alone": does authority rest on a human lineage, or on text and consensus? Each great tradition offers a different solution.
The power of martyr-memory. The Shia tradition of mourning built around Husayn's martyrdom invites comparison with the Christian narrative of the Passion—the sacrifice of one killed by unjust power becoming a moral compass and identity-core for an entire community. Scholars of comparative religion often use this to discuss how "martyrdom" shapes collective memory.
Tensions today. The contemporary Sunni–Shia relationship is often swept up in the state rivalries and geopolitical contests of the Middle East; understanding this divide requires keeping seventh-century theological history distinct from twenty-first-century political reality—related, but not identical. The next installment (Day 8) turns to Sufism, the mystical tradition that runs across both.
This piece introduces the Sunni and Shia traditions from a neutral, comparative standpoint, aiming to present the internal logic of each without praise, blame, or taking sides, and without judging any living religious figure. Names and historical details follow common scholarly accounts; dates are often traditional or estimated; population figures are recent estimates and are given as ranges because definitions and methods differ.