Linking the Pacific Ocean to the Indian Ocean, Southeast Asia has long been an important passage for trade. Hot on the heels of traders were missionaries and proselytizers. The first of world religions to arrive in the region were Hinduism and Buddhism via trade links with India. Indian merchants not only established trading stations along the region but also brought along their religious and cultural influences. Beyond the spiritual, Hinduism and Buddhism have contributed to the development of a written tradition in Southeast Asia. And while Southeast Asian converts have incorporated local features and cultures into Hinduism and Buddhism for a distinct flavour, aspects of their religious life are still observably Indian. By the twelfth century, peoples in what are known today as Myanmar, Thailand, Laos, Cambodia, and Vietnam were converted to Buddhism. In Bali, part of Muslimdominated Indonesia, approximately 90 per cent of the population practise Hinduism.

The year and source of Islamic influence in Southeast Asia is a matter for debate. Some European scholars argue that Islam entered the region upon trade with India, some regional Muslim historians assert that it was imported directly from the Middle East, while other scholars believe that Muslim Chinese introduced it when they traded in the region. Nonetheless, it is generally accepted that Islamic teachers had, by the fifteenth century, established religious schools largely in the Malay Peninsula as well as in parts of Sumatra, Java, Kalimantan, and Mindanao. In the course of time, Islam absorbed many pre-existing Southeast Asian beliefs such as animism. Presently both Malaysia and Indonesia are predominantly Muslim, with the latter the most populous Muslim country in the world.

Christianity was, comparatively speaking, a late arrival to Southeast Asia. Christian missionaries only ventured into the region in significant numbers with the spread of colonialism from the eighteenth century. Failing to penetrate China and Japan, many Christian missionaries settled along Southeast Asia.