Islam is Indonesia's dominant religion and approximately 86%, or about 200 million, of its population identify as Muslims, making it the world's largest Muslim population.
The Indonesian Central Statistic Bureau (BPS) conducts a census every 10 years. The latest data available, from 2000, indicated that of 240,271,522 people, 86.1 percent of the population label themselves Muslim, 5.7 percent Protestant, 3 percent Catholic, 1.8 percent Hindu, and 3.4 percent "other or unspecified". The country's religious composition is a politically charged issue, with some commentators saying that the census perennially undercounts non-Muslims.
Most Muslims are Sunni, although some follow other branches of Islam. Shia number around one million. In general, the Muslim community can be categorized in terms of two orientations: "modernists," who closely adhere to orthodox theology while embracing modern learning; and "traditionalists," who tend to follow the interpretations of local religious leaders (predominantly on Java) and religious teachers at Islamic boarding schools.
The first Indonesians to adopt Islam are thought to have done so as early as the eleventh century, although Muslims had visited Indonesia early in the Muslim era. The spread of Islam was driven by increasing trade links outside of the archipelago; in general, traders and the royalty of major kingdoms were the first to adopt the new religion. Dominant kingdoms included Mataram in Central Java, and the sultanates of Ternate and Tidore in the Maluku Islands to the east.
By the end of the thirteenth century, Islam had been established in North Sumatra; by the fourteenth in northeast Malaya, Brunei, the southern Philippines and among some courtiers of East Java; and the fifteenth in Malacca and other areas of the Malay Peninsula. Through assimilation Islam had supplanted Hinduism and Buddhism as the dominant religion of Java and Sumatra by the end of the 16th century. At this time, only Bali retained a Hindu majority and the outer islands remained largely animist but would adopt Islam and Christianity in the seventeenth and eighteenth centuries.
Despite being one of the most significant developments in Indonesian history, historical evidence is fragmentary and generally uninformative such that understandings of the coming of Islam to Indonesia are limited; there is considerable debate amongst scholars about what conclusions can be drawn about the conversion of Indonesian peoples. The primary evidence, at least of the earlier stages of the process, are gravestones and a few travellers' accounts, but these can only show that indigenous Muslims were in a certain place at a certain time.
This evidence cannot explain more complicated matters such as how lifestyles were affected by the new religion or how deeply it affected societies. It cannot be assumed, for example, that because a ruler was known to be a Muslim, that that the process of Islamisation of that area was complete; rather the process was, and remains to this day, a continuous process in Indonesia. Although it is known that the spread of Islam began in the west of the archipelago, the fragmentary evidence does not suggest a rolling wave of conversion through adjacent areas; rather, it suggests the process was complex and slow.In the late fifteenth century, the powerful Majapahit Empire in Java was at its decline. After it had been defeated in several battles, the last Hindu kingdom in Java fell under the rising power of Islamized state Sultanate of Demak in 1520. Islam in Java then began to spread formally, largely influenced by the Wali Songo (or the Nine Saints).
Muslims constitute a majority in most regions of Java, Sumatra, West Nusa Tenggara, Sulawesi, coastal areas of Kalimantan, and North Maluku. Muslims form distinct minorities in Papua, Bali, East Nusa Tenggara, parts of North Sumatra, most inland areas of Kalimantan, and North Sulawesi. Together, these non-Muslim areas originally constituted more than one third of Indonesia prior to the massive transmigration effort sponsored by the Suharto government and recent spontaneous internal migration.
Internal migration has altered the demographic makeup of the country over the past three decades. It has increased the percentage of Muslims in formerly predominantly Christian eastern parts of the country. By the early 1990s, Christians became a minority for the first time in some areas of the Moluccas. While government-sponsored transmigration from heavily populated Java and Madura to less populated areas contributed to the increase in the Muslim population in the resettlement areas, no evidence suggests that the Government intended to create a Muslim majority in Christian areas, and most Muslim migration seemed spontaneous. Regardless of its intent, the economic and political consequences of the transmigration policy contributed to religious conflicts in Maluku, Central Sulawesi, and to a lesser extent in Papua.
The leading national "modernist" social organization, Muhammadiyah, has branches throughout the country and approximately 30 million followers. Founded in 1912, Muhammadiyah runs mosques, prayer houses, clinics, orphanages, poorhouses, schools, public libraries, and universities. On February 9, Muhammadiyah's central board and provincial chiefs agreed to endorse the presidential campaign of a former Muhammadiyah chairman. This marked the organization's first formal foray into partisan politics and generated controversy among members.
Nahdlatul Ulama (NU), the largest "traditionalist" social organization, focuses on many of the same activities as Muhammadiyah and indirectly operates a majority of the country's Islamic boarding schools. Claiming approximately 40 million followers, NU is the country's largest organization and perhaps the world's largest Islamic group. Founded in 1926, NU has a nationwide presence but remains strongest in rural Java. The Islam of many NU followers has heavy infusions of Javanese culture, and followers tend to reject a literal or dogmatic interpretation of Islamic doctrine. Many NU followers give great deference to the views, interpretations, and instructions of senior NU religious figures, alternately called "Kyais" or "Ulama." The organization has long advocated religious moderation and communal harmony.
A number of smaller Islamic organizations cover a broad range of Islamic doctrinal orientations. At one end of the ideological spectrum lies the controversial Islam Liberal Network (JIL), which aims to promote a pluralistic and more liberal interpretation of Islamic thinking. Equally controversial are groups at the other end of this spectrum such as Hizbut Tahrir Indonesia (HTI), which advocates a pan-Islamic caliphate, the Indonesian Mujahedeen Council (MMI), which advocates implementation of Shari'a as a precursor to an Islamic state, and the sometimes violent Front Pembela Islam (FPI). Countless other small organizations fall between these poles.
Separate from the country's dominant Sunni Islam population, a small minority of persons subscribe to the Ahmadiyah interpretation of Islam. However, this group maintains 242 branches throughout the country. In 1980 the Indonesian Council of Ulamas (MUI) issued a "fatwa" (a legal opinion or decree issued by an Islamic religious leader) declaring that Ahmadiyah is not a legitimate form of Islam.
In addition there are small numbers of other messianic Islamic groups, including the Malaysian-affiliated Darul Arqam, and the syncretist Indonesian Jamaah Salamulla group (also called the Salamulla Congregation or God's Kingdom). Its leader, Lia Eden, is currently facing charges of disdaining Islam and many Islamic organizations in Indonesia consider them as a heretical form of Islam.