Mongolian religion has two roots and one catastrophe. The older root is a sky centred practice addressed to Tenger, to mountain and river masters and to ancestors, marked by the ovoo cairns still built on passes and circled three times by travellers, and conducted by shamans whose drums and rituals are attested from the earliest sources on the steppe. The second root is Tibetan Buddhism.
Kublai Khan patronised the Sakya lama Phagpa in the thirteenth century, but the decisive conversion came in 1578 when Altan Khan met Sonam Gyatso and conferred on him the title Dalai Lama, the Mongolian word for ocean, retroactively applied to his two predecessors. Gelug Buddhism then became the dominant institution of Mongolian life, and by 1900 a very large proportion of Mongolian men were monks and the monasteries held much of the country's wealth and nearly all of its literacy.
The Bogd Khan, the eighth Jebtsundamba Khutuktu, was both religious head and, from 1911, head of state. The catastrophe was the purge of 1937 and 1938, in which the communist government under Khorloogiin Choibalsan closed or destroyed nearly every one of some seven hundred monasteries and killed clergy in the tens of thousands.