You will have to be specific about the country because Southeast Asia is a culturally and ethnically diverse part of the world.
The main religions include Christianity in the Philippines, parts of Indonesia and parts of Malaysia. There are Christians in East Timor as well. Buddhism is predominant in Vietnam, Thailand, Laos, Cambodia, Burma and Singapore. Confucianism and Ancestor worship is practiced in Vietnam and Singapore. Islam is dominant religion in Indonesia, Brunei, Malaysia, Southern Thailand and in Southern Philippines.