Islam in Indonesia — The World's Largest Muslim Population

Indonesia is home to roughly 240 million Muslims — about 87% of the national population, and more than any other country in the world. That single fact makes Indonesia one of the most important countries in the Islamic world, despite being culturally and geographically far from the Middle Eastern centres most foreigners associate with Islam. Understanding how Islam works in Indonesia — how it arrived, how it has been shaped by local context, and how the major organisations operate — is essential context for almost everything else about the country.

How Islam arrived

Islam came to the Indonesian archipelago slowly and peacefully through trade, beginning in the 12th and 13th centuries. The earliest converts were in the northern Sumatran ports — Aceh especially — where Arab, Persian, and Indian Muslim traders had long worked. From the coastal trading cities, Islam spread inland and eastward over the following four centuries, often through the agency of Sufi missionaries and through the conversion of local rulers whose subjects then followed.

This pattern of voluntary, gradual, mercantile conversion is fundamentally different from the conquest-driven spread of Islam across the Middle East and North Africa. There was no Islamic caliphate ruling Indonesia, no Arabic-speaking elite imposing Sharia on a non-Muslim population. The result is an Islam that absorbed and built on top of the pre-existing Hindu-Buddhist and animist cultures, rather than replacing them.

The conversion of Java was largely completed in the 15th and 16th centuries through the work of the Wali Songo — the nine saints of Javanese Islam. These figures, most legendary in nature but some historical, used culturally syncretic methods: gamelan music, wayang shadow puppetry, and adaptations of Hindu epics to teach Islamic ideas. Their tombs are pilgrimage sites today.

The syncretic layer

What emerged in much of Indonesia, especially on Java, is what scholars call syncretic Islam. The orthodox doctrines and ritual practices are intact — the five daily prayers, the Ramadan fast, the pilgrimage, the testament of faith. But layered on top, and often interwoven, are pre-Islamic practices: visits to ancestral graves, offerings at sacred sites, consultations with dukun (traditional healers and diviners), pilgrimages to the tombs of the Wali Songo, kejawen mystical practice.

The American anthropologist Clifford Geertz, writing in the 1950s, distinguished three Javanese Muslim styles: santri (orthodox, mosque-going, religiously rigorous), abangan (nominally Muslim but heavily syncretic), and priyayi (the aristocratic, mystically inclined court tradition). The categories are dated and have been criticised, but they still capture something real about the variation in how Indonesian Muslims actually practise their religion.

Outside Java, the pattern varies. Acehnese Islam is much more orthodox and Sharia-influenced than Javanese. Minangkabau Islam is reformist. Coastal Sumatran and Sulawesian Islam often shows stronger Sufi influences. Eastern Indonesian Islam, where Muslims live alongside Christian and Hindu populations, is often more cosmopolitan.

The major organisations

Indonesian Islam is not a centralised church, but it is unusually well-organised. Two mass organisations — the largest of their kind in the world — shape religious and political life:

Nahdlatul Ulama (NU) is the larger, founded in 1926 by traditionalist clerics. NU has perhaps 90 million members and is associated with traditional Sunni jurisprudence, the network of pesantren (Islamic boarding schools), the syncretic Javanese tradition, and a generally tolerant, pluralist stance in national politics. Its political wing has historically been the PKB. Abdurrahman Wahid (Gus Dur), Indonesia's fourth president, was an NU figure.

Muhammadiyah, founded in 1912, is the more modernist organisation. It has about 60 million members and is associated with reformed, scripture-focused practice; a network of universities, hospitals, and schools; the Minangkabau and urban middle-class membership; and a politically active but generally moderate stance. The Amien Rais-founded PAN was historically its political vehicle.

The combination of these two mass organisations — about 150 million members across both — gives Indonesian civil society a backbone that does not exist in most other Muslim-majority countries. Both NU and Muhammadiyah operate vast educational, health, and welfare systems independently of the state.

There are also smaller, more conservative organisations — including hardline groups associated with Salafi or Islamist tendencies. These have grown in influence since the 1998 Reformasi but remain a small minority of the Muslim population overall.

Politics

The relationship between Islam and the state in Indonesia is complex and historically contested.

The 1945 Constitution did not establish Islam as the state religion, despite intense debate at independence. The compromise was Pancasila — the five principles, the first of which is "belief in the one supreme God" without specifying which religion. Indonesia is constitutionally a religious state, not a secular one, but it is also not an Islamic state.

The state recognises six official religions: Islam, Protestantism, Catholicism, Hinduism, Buddhism, and Confucianism. Citizens are required to declare a religion on their identity cards (a constitutional court ruling in 2017 allowed traditional beliefs as a seventh category).

Sharia law applies only in Aceh, by special arrangement following the 2005 peace agreement, and even there only to Muslims and in limited domains (criminal law for offences like gambling, public alcohol, and unmarried relationships; family law). Elsewhere, Indonesian civil law applies regardless of religion.

The political balance between secular, religious-tolerant, and more conservative Islamic positions has shifted over the decades. The Reformasi period saw a rise in conservative Islamic political organising, with episodes like the 2017 protests against then-Jakarta governor Ahok (a Chinese Christian) being widely interpreted as a turning point. The Jokowi years saw the government push back against the most hardline groups while accommodating mainstream Islamic political demands.

Daily life

For visitors, the visible signs of Islam are pervasive but rarely intrusive.

The call to prayer (azan) sounds five times a day from mosques in every city, town, and village. The dawn call (about 4:30am) is the most likely to wake you. The Maghrib call at sunset is the most pleasant — the moment the city pauses briefly.

Friday prayer (about 12:30–1:30pm) empties offices, shops, and government buildings of male staff for an hour. Lunch hours in offices extend on Fridays.

Ramadan, the month of fasting, has a significant impact on daily life — restaurants in non-tourist areas often close during the day, traffic patterns shift, work pace slows, and the evening iftar (break-fast) becomes the day's social centre. The dates shift by about 11 days earlier each year on the Western calendar.

Idul Fitri (the end-of-Ramadan holiday) and Idul Adha (the festival of sacrifice) are both major public holidays. Idul Fitri produces mudik — the annual mass migration of urban workers back to their home villages — and is the busiest travel time of the year.

Dress: most Indonesian Muslim women wear hijab, but rules vary by region and individual. In tourist Bali, in Jakarta's cosmopolitan districts, and at universities, you'll see a full range from hijab to no head covering. In Aceh, conservative dress is universal and visitors should follow suit. In rural and traditional areas across the rest of Indonesia, modest dress is appreciated.

Visiting mosques

Most large mosques welcome visitors outside prayer times. Standard etiquette:

• Remove your shoes.
• Cover your head if you're a woman (a scarf is usually available at the entrance).
• Wear long sleeves and long trousers / a long skirt.
• Don't enter the prayer hall during prayer unless you're praying.
• Don't take photos of people praying.

The Masjid Istiqlal in Jakarta (the largest mosque in Southeast Asia, holds 200,000), the Great Mosque of Demak (one of the oldest in Java, attributed to the Wali Songo), and the Masjid Raya Baiturrahman in Banda Aceh are particularly impressive examples.

Why this matters

For a visitor, the importance of understanding Indonesian Islam goes beyond cultural courtesy. The religion shapes the rhythm of daily life, the timing of holidays and travel, the dress codes, the food (halal is the default), and the social structure of every workplace, school, and neighbourhood you'll interact with. It also shapes Indonesian politics in ways that affect everything from foreign investment rules to women's rights to LGBTQ+ visibility.

The contrast with Middle Eastern or South Asian Islam is real and important — Indonesian Islam is its own thing, with its own history, organisations, and internal variation. Treating it as homogeneous misses most of what makes it distinctive.