To various tribal communities like the Sonowal-Kacharis, Tiwas, Karbis and the Dimasas, Siva is one of the principle deities and is called by different tribal names. The Bodo-Kacharis believe that they are descendants of Bathau-Sivrai, which is one of the manifestations of Siva. The ruling family of Tripura used to claim their origin from Siva.
Ancient Assam was recognized as the principal center of Sakta cult with its chief temple at Kamakhya. The Kalika Puran was composed to glorify the goddess Kamakhya. In Assam there were at least four other sites of Sakta worship including Ugra Tara in Guwahati, the copper temple in Sadiya. In ancient texts Siva is invariably mentioned whenever there is a mention of Devi worship. However Tara, Candi etc are also Buddhist Gods and it is imputed that Sakta worship degenerated (10th century) due to its association with occult Buddhism. The cult of Vishnu regained royal patronage.
The worship of Sakti did not entirely subside. The worship of the Devi involved animal sacrifice and had an appeal to royal families because of the power said to have derived with such bloodshed. The Koch rulers in western Assam, Chutiya kings of Sadiya, The Kacharis and the Ahoms were true Sakta devotees. Sakta worship still continues in various temples of Assam today. There are folk variations of this worship related to the snake God Manasa, which continues till today mainly in Western Assam.
Vishnu was a God subsidiary to Siva and evolved due to the preponderance of Brahmans who wanted to curb the liberalism in Siva worship. Several Sanskrit scholars whose works flourished in Kamrupa between the 11th and 12th centuries were Vaishnavites in religious creed. The cult of Vishnu regained popularity and royal patronage due to the debasement of Saktism from its fusion with esoteric Buddhism. Vaishnavism was however highly ritualistic and priest ridden. The religious rites were conducted in Sanskrit language and were not found suitable by the non-Aryan ethnic groups.
The Kamarupa King Bhaskaravarman had traveled to Kanauj and Prayag (Allahbad). He had been with Emperor Harsha at Prayag at the festival that took place every five years in the Ganga-Yamuna confluence. Neither they or any other monks whether Buddhists, Jain, Ajivikas or others who gathered there in thousands along with the Brahmins for a great distribution of gifts by the Emperor Harsha would have been conscious of any contradiction. This incongruity of the Indian character had become current even in Kamrupa before Hueng Tsang's (625 AD) arrival at Guwahati. Hueng Tsang had noted this incongruity where on the one hand 'whatever Buddhist there was in Kamarupa they performed their acts of devotion secretly.' While on the other 'the King Bhaskaravarman though not a Buddhist treated accomplished Sramanas with respect". Bhaskaravarman had told the Chinese pilgrim that if he settled in his dominion he would build 100 monasteries on his behalf.
Buddhism had entered the valley definitely by the 1st century AD. Perhaps some of its esoteric practice was regarded as subversive during the time of Hueng Tsang's visit. Around this time Buddhism was under a cloud in UP and Bihar. The Kamarupa king's adversary Sasanka of Gouda was reputed to have destroyed many Buddha images besides cutting down the Bodhi tree at Gaya. In 655 AD Nalanda monastery was sacked and burnt down in the general disorder after the death of Harsha.
In Kamarupa after the 7th century AD there is convincing evidence of the prevalence of Vajrayana or Tantric Buddhism. Recent researches have indicated this more convincingly. (See Links). The circular shrine in Chaygaon near Guwahati locally known as the Medh is yet another vestige of Buddhism. The placement of a circular slab under ground level and encircling it by a circular passage with a rectangular exit (See photo) indicates the antiquity of this shrine. The finds near Narakasur Hill near Guwahati, which are now displayed in the State Museum, include two pieces of Bodhisattva Avalokitesvara, two pieces of Tara and one of Canda. (see photos) However the Brahmans counterpoised a Sakta Tantricism in such a manner that the distinction between the two creeds became diffused. Tantric Buddhism waned but an occult debased form of Tantric Buddhist magic cults continued for a while.
The Tai-Ahoms were partly Hinayana Buddhists partly Tantric Buddhists having their own form of ancestor worship. During the early phase of their existence in the valley they performed the Phura lung Puja which is identified with Buddhists traditions. In the 18th century Hinayana Buddhism migrated from Burma to Assam with the Khamptis. The Khamptis were a group of Shans who were allowed to settle in Sadiya by the Ahom rulers. The Singphos also came from Burma (1793) and pais lip-service to Buddhism. Earlier (1760) many groups of the Buddhist faith like the Tai-Phakes, Nara, Turungs and Aitaniyas trickled into Assam. The Sherdukpens, Monpas and Khambas of Arunachal Pradesh have been professing Tibetan Buddhism for nearly two centuries now.