Islam in Southeast Asia - WiKi
According to the Islamic Council of Victoria, historians argue "that by the beginning of the ninth century Arab merchants and sailors, (and other Muslims) had begun to dominate Nanhai (Guangzhou) or Southeast Asian Trade." There existed a colony of foreign Muslims on the west coast of Sumatra by 674 CE; other Muslim settlements began to appear after 878 CE when Islam increasingly took firm root among the people. The religion did not spread to significant parts of the population until the 12th century.
In the 12th century, a turbulent period occurred in the history of Maritime Southeast Asia, the Indian Chola navy crossed the ocean and attacked the Srivijaya kingdom of Sangrama Vijayatungavarman in Kadaram (Kedah). The capital of the powerful maritime kingdom was sacked and the king was taken captive. Along with Kadaram, Pannai in present day Sumatra and Malaiyur and the Malayan peninsula were attacked. Soon after, the King of Kedah Phra Ong Mahawangsa became the first ruler to abandon the traditional Hindu faith, and converted to Islamwith the Sultanate of Kedah established in year 1136. Samudera Pasai converted to Islam in the year 1267. The King of Malacca Parameswara married the princess of Pasai, and their son became the first sultan of Malacca. Soon Malacca became the center of Islamic study and maritime trade; other rulers followed suit. Indonesian religious leader and Islamic scholar Hamka(1908–1981) wrote in 1961: "The development of Islam in Indonesia and Malaya is intimately related to a Chinese Muslim, Admiral Zheng He."[1]
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.