Although Muslim traders first traveled through South East Asia early in the Islamic era, the earliest evidence of Islamized populations in Indonesia dates to the 13th century in northern Sumatra. 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 complicated and slow. 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.
Other Indonesian areas gradually adopted Islam, making it the dominant religion in Java and Sumatra by the end of the 16th century. For the most part, Islam overlaid and mixed with existing cultural and religious influences, which shaped the predominant form of Islam in Indonesia, particularly in Java. Only Bali retained a Hindu majority. In the eastern archipelago, both Christian and Islamic missionaries were active in the 16th and 17th centuries, and, currently, there are large communities of both religions on these islands.