In its known history Tibet has been host to two principal religions: Mahāyāna Buddhism (in its Vajrayāna aspect), which is now represented by the four major schools of Nyingma, Sakya, Kagyu, Geluk, and the indigenous Bön religion, also now divided into different schools, which has come to resemble Tibetan Buddhism in most respects.
The first Tibetan king truly to adopt Buddhism was Trisong Detsen (c.741–97), the ‘Tibetan Aśoka’. He built the first Tibetan monastery at Samyé, invited the Indian missionaries Śāntarakṣita, Kamalaśīla, and Padmasambhava, and ensured Tibetan Buddhism would develop along Indian rather than Chinese lines. Perhaps in response to this as much as in defence of the native Bön, King Langdarma (c.803–42) seized power from King Ralpacan (c.805–36), and began a programme of persecution of Buddhism so ruthless that historians now isolate ‘first and second diffusions’ of Buddhism in Tibet. Langdarma was himself assassinated by a Buddhist monk, propelling the country into two centuries of anarchy.
The ‘second diffusion’ began in the 11th cent. In 1012 the heirs of those monks who had escaped to the east moved into central Tibet and founded the Gyal Lukle monastery. Simultaneously, sympathetic descendants of the broken royal line which had escaped to the west encouraged translators such as Rinchen Zangpo (c.958–1055) to study with Indian teachers and bring back sūtras and tantras. The influence of Atisa, who arrived in 1042, has endured to the present. For the next nine centuries Tibetan Buddhism developed its own character. It may have assimilated something of the native Bön tradition, but it owes far more to Indian Vajrayāna.
As can be seen from the biographies of the Dalai and Panchen Lamas, Tibet has paid for its own pre-Buddhist imperialism by relentless cycles of Mongolian and Chinese invasion. It had survived with its independence and religion unscathed until the 1950 annexation by China and the driving into exile of the present Dalai Lama. After that, the systematic dismantling of Tibetan religion and culture began. The accusation of genocide against China by the International Commission of Jurists in 1960 had no effect in obstructing the policy, and by the time Mao died in 1976, 1.2 million Tibetans are estimated to have died as a result of the occupation.
Today, religious activity (banned completely 1966–79 along with the use of Tibetan clothing and even food bowls) remains subject to strict controls. A few monasteries (on tourist routes) have been renovated, and a limited number of monks (who must be vetted by the Communist Party committees governing each monastery) are now permitted. Though monastic education is reappearing, the use of tantric imagery remains subject to stringent restrictions (this has been likened to a ban on representing Mary in Catholicism), and it is outside Tibet that Tibetan religions currently flourish, with a particular growing appeal for Tibetan Buddhism in the West.
"Tibetan religion." The Concise Oxford Dictionary of World Religions. . Encyclopedia.com. (August 21, 2018). http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/tibetan-religion
"Tibetan religion." The Concise Oxford Dictionary of World Religions. . Retrieved August 21, 2018 from Encyclopedia.com: http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/tibetan-religion