Prior to the arrival of the Thai people, the central and peninsula area of what would become Siam, along with the neighbouring lands of Cambodia, southern Laos and southern Vietnam were part of the ancient Funan Kingdom, dating from the first century BC, whose peoples followed a concoction of ancient traditional animist beliefs mingled with the traditions of India, an influence which widely spread throughout the region, bringing both Hindu and, later, Buddhist practices into play.

When the kingdom declined and dissolved, the area was shared between the Mon, who occupied present day Myanmar (Burma) and the Khmer Chenla civilisations, before they too were in turn subsumed by the rise of the Khmer Empire of Angkor, which established control from 979 AD, with Hinduism as its dominant ideology.

The precise origins of the Thai peoples is still not a settled matter amongst historians, and many competing theories still jostle for prominence. What is known is that they migrated from southern China during the eighth and ninth centuries AD, and became vassals and even slaves of the Khmer Empire, as is detailed in carvings found at Angkor Wat.

As Angkor began its decline, and following further southward migration into what is now central and northern Thailand, the Thai peoples were able to establish themselves within the eastern Khmer empire and the emergent Kingdom of Lavo around the Chao Phraya valley.

By 1238, the Thais had managed to wrest themselves from Khmer control and were finally able to found a city state of their own, at Sukhothai, under their first king, Sri Indraditya, a distinctly Indian appellation attesting to the prevalence of the Hindu tradition of the time.

Flowing his death, Sukhothai expanded further, and adopted Theravada Buddhism as the state religion, which has underpinned Thai society ever since, though elements of Hindu Brahmanism and Chinese folk beliefs, including spirit and ancestor worship, together with Taoism and Confucianism, which had survived their migratory period, still colour the practice of Buddhism in the modern day country. Hinduism itself is still practiced as a minority religion in Thai society.

The Sukhothai state would eventually fall to the rival Thai Kingdom of Ayutthaya, which had also adopted Theravada Buddhism, and from this time the nation of Siam as we know it today would emerge and consolidate.  

From the Sukhothai period onwards, Arab traders also began to bring the influence of Islam into Thai culture, and the later Ayutthaya kingdom would host Islamic minorities from Persia and Malaya to the south. Islam remains a minority religion in Thailand, but is practised all over the country.

While Islam, which is also infused with Sufism, is well respected and tolerated in Thailand, to the far south of the peninsula, a certain degree of conflict characterises the regions of Patani, Yala and Narathiwat, former Islamic Sultanates incorporated into Thailand during the historic definition of the border by the Anglo-Siamese Treaty of 1909, at a time when Malaya was ruled by the British.

As a consequence, in more recent times, beginning in 1948, a separatist insurgency has begun to agitate the Thai government, a movement which became more serious from 2001, when Jihadi extremist influences began to penetrate the struggle.

Christianity first appeared in 1550, at the hands of Portuguese missionaries who arrived in Ayutthaya and established a small presence, and since that time several waves of Christian missionaries, both Catholic and Protestant have been welcomed with tolerance.

Perhaps because, unlike its near neighbours in Indochina, Thailand was never colonised by European powers, resistance to Christianity was less marked, and its minority influence continues to prevail and indeed is often credited with having contributed positively to Thai modernisation.

Other minority religions practised in Thailand include Sikhism and Judaism.

In the modern era, the Thai constitution provides for freedom of belief, subject to their organisations being registered with the government, though in practice many unregistered denominations operate largely unhindered.

Although not specifically designated as a state religion, it is a very serious offence, enforced with harsh penalties, in Thai Law even to casually mock or offend Buddhism or the Thai royalty and the constitution explicitly stipulates that the ruler must be a Buddhist.