There are several theories to the Islamization process in Southeast Asia. The first theory is trade. The expansion of trade among West Asia, India and Southeast Asia helped the spread of the religion as Muslim traders brought Islam to the region. The second theory is the role of missionaries or Sufis. The Sufi missionaries played a significant role in spreading the faith by syncretising Islamic ideas with existing local beliefs and religious notions. Finally, the ruling classes embraced Islam which further aided the permeation of the religion throughout the region. The ruler of the region's most important port, Malacca Sultanate, embraced Islam in the 15th century, heralding a period of accelerated conversion of Islam throughout the region as the religion provided a unifying force among the ruling and trading classes.
Reference: wikipedia (2015) Islam in Southeast Asia. Available from < https://en.wikipedia.org/wiki/Islam_in_Southeast_Asia > [Mar 3 2016]