What were the religious beliefs of the Mongols?

The Mongols were Tengerians, which is a kind of shamanistic belief system. Shamanism is a religious practice in which a holy man or shaman is believed to have direct access to the spirit world. Through this interactions with the spirit world, the shaman is supposed to derive great wisdom, which he then passes on to his people. This spirit-derived wisdom then forms the basis of the relevant society's cultural practices.
The Mongols' Tengerism was also a species of animism, in that it honored and venerated spirits such as Koke Mongke Tengri, the Eternal Blue Heaven, and Mother Earth. There were three main components to Tengerism: the proper care and veneration of the spirits; the cultivation of personal responsibility; and the maintaining of an appropriate balance between the individual, the spirits, and the community. Any catastrophe that befell the Mongols, such as crop failure or defeat in battle, was attributed to a lack of balance, and in such circumstances a shaman would be called in to try and get to the heart of the problem by communing directly with the spirits.