Since the ancient time, the Khmers had accepted the two great religions from India, i.e. Hinduism and Buddhism, by which the former one was more popular. Hinduism had played an important role in Khmer civilization as the Angkor monarchs adopted its concept of deva-raja, or "god-king", by which the king was revered as an incarnation of the god Shiva, a supreme Hindu deity who was regarded as a protector.
Most temples in the Angkor Empire were dedicated to either god Shiva or god Vishnu. Believed to be the holy house of the supreme gods, the temples were carefully built with fine arts, and the materials used are those of everlasting stones. Many impressive sculptures of great craftsmanship were enshrined.
The second religion being revered by the Khmers was Buddhism of Mahayana sect which came into the region quite at same time as
Hinduism, however, Buddhism was less prominent.
Both Hinduism and Mahayana Buddhism played an important role as the political, religious and philosophical pillars of Khmer Civilization by which the king was revered as the god-king or deva-raja. This ideology enabled the king to rule over the country as an absolute monarch with sovereign spirituality over his people, and thus enhanced the unity of the kingdom. Successive kings were able to mobilize large manpower to serve the army, to maintain extensive irrigation system and to build numerous massive temples.
Not until the 13th century, Theravada Buddhism was introduced to the Khmer from Sri Lanka and became more prominent in the royal court as well as in the local people. The teaching of Theravada Buddhism directly crashed with the original belief of the Khmer people as it taught the people to seek self enlightenment and abandoned the worldly things. With this teaching, the attitudes of the people towards its Hindu gods as well as the god-king changed, and thus led to the gradual weakening of the empire which eventually collapsed in the first half of 15th century.
The Khmer people seem to be the obedient students, as they did not raise doubts about the riligious teaching of the original doctrines. We can see in Khmer history that the religions such as Hinduism and Buddhism were not divided into the different sub-sects in the land of Khmer, as they were so in some other civilizations.
In addition to Hinduism and Buddhism, the Khmer people also had their own indigenous beliefs such as the local deities, ancestral spirits, as well as the evil spirits. There are no inscriptions or manuscripts to describe these beliefs, however, it can be found to be prevalent in modern Cambodia, especially in the remote villages. These beliefs are passed on from one generation to another through the words of mouth.