Mahayana (Tibetan) Buddhism is the dominant religion in Mongolia, although shamanist beliefs still exist exist and may be practiced side by side with it.
In the 13th Century Kublai Khan, in his court in Khanbalig (Beijing) came into contact and was influenced by Buddhism when he met Drogön Chögyal Phagpa, leader of the Sakya school of Tibetan Buddhism who became his guru and spiritual advisor; but it was 1578 when Altan Khan met a Tibetan spiritual leader called Sonam Gyatso, was converted to Buddhism and gave him the title "Dalai Lama" ("Dalai" meaning "ocean" in Mongolian and "Lama" meaning "teacher" in Tibetan): this was the spark that reintroduced Tibetan Buddhism to Mongolia and mass conversions followed. Men and boys were conscripted to monastery life and as a result there was far less fighting and armed conflict with China, that encouraged this by funding more monasteries.
Over the centuries monasteries increased their wealth and power and in the early twentieth century, Outer Mongolia had 583 monasteries and temple complexes, controlling an estimated 20% of the country's wealth. Almost all Mongolian cities grew up on the sites of monasteries. Ulaanbaatar, for example, had two monasteries with around 13,000 and 7,000 monks. When the People's Republic was formed, the revolutionaries were determined to modernise the country and felt the Buddhist church was in the way; in 1934 there were well over 800 major Buddhist centres and 3,000 temples, an annual income that almost matched that of the state, while more than 13% of Mongolia's population was a monk. Following Stalin's example of prosecuting the Orthodox church, Khorloogiin Choibalsan initiated a campaign of terror and destruction in the late 1930s, almost all monasteries and temples destroyed, up to 30,000 monks massacred with thousands more sent to Siberian labour camps.
Some buildings were retained as museums, like the three temples in Erdene Zuu monastery near the old Mongol capital of Karakorum, Zayain Gegenii Süm, now the Arkhangai Provincial Museum in Tsetserleg and the Choijin Lama Temple museum in the heart of Ulaanbaatar. Only one monastery, with a community of 100 monks, remained open: the Gandantegchinlen (Gandan) Monastery, established in 1835 by the Fifth Jebtsundamba, then Mongolia's highest reincarnated lama and the principal centre of Buddhist learning in Mongolia. It became a display monastery for foreign visitors: when the American Vice President, Henry Wallace, visited Mongolia in 1944 and asked to see a monastery he was taken here. Gandan remained a "show monastery" for other foreign visitors until 1990 when it was restored to full functionality and today has more than 600 monks. All over Mongolia monasteries and temples have been restored and rebuilt and although two generations have been raised without being exposed to Buddhist ritual, there is a revival and some young boys are again trained as monks.