Tengrism

Tengri, the Eternal Blue Sky, appears in Chinese chronicles as the sky god of the Xiongnu in the fourth century BC. The Secret History of the Mongols, written in 1240, begins with Borte Chino, the Blue Wolf, born with a destiny from Tengri. In 1290 the Ilkhan Arghun told Pope Nicholas IV that the descendants of Genghis Khan, whether they accepted baptism or not, left that for Eternal Tengri to decide.

Tengrism's hold lasted until the fourteenth century, when the Golden Horde converted to Islam. After that it was largely submerged, and among the Mongols it lived on mixed with Tibetan Buddhism.