Sukhothai's King Ramkhamhaeng (1275-1317 A.D.) established Theravada Buddhism as Thailand's dominant religion. It reached its height under the reign of King Ramkhamhaeng's grandson, king Li Thai (1347-1368 A.D.), when about 30 volumes of the Buddhist scriptures were studied and rewritten by the king into one volume, the Tribhumikatha, a treatise on Buddhist cosmology and the three planes of existence-Sensuous, Corporeal, and Incorporeal. Not only was this the first Buddhist treatise by a Thai, but it was this the first known Thai Buddhist literary work.

Through the centuries Buddhism has been the main driving force in Thai cultural development. Much of classical Thai art, particularly architecture, sculpture, painting, and early literature, is really Buddhist art. Then and even now, Buddhism colors everyday Thai life.

Although Buddhism became the primary and state religion, Thais always subscribed to the ideal of religious freedom. Thai constitutions have stipulated that Thai king must be Buddhists, but monarchs are invariably entitles "Upholder of All Religions". Consequently, the government, through the Religious Affairs Department, annually allocates funds to finance religious education
and to construct, maintain, and restore monasteries, mosques, and churches.