Toghrul Quan of the Keraits, and most of the Keraits themselves were Nestorian Christians. So was Hulegu’s wife.
According to the Syrian Bar Hebraeus, the Kerait were converted to Nestorianism, a sect of Christianity, about the year 1008.
SNIP
During the 10th and 11th centuries, due to Nestorian missionary activities, several Turkic tribes were entirely or to a great extent Christian, notably the Kerait, Uyghur, Naiman and Merkit. They were a cluster of hunting tribes east and south of Lake Baikal. The principal tribes evangelized there by the Nestorians were the Naiman, the Merkit, and the Kerait. The Kerait capital at this time was Karakorum, where Marco Polo later found a church.
It seems that the Gospel was taken to those tribes by Christian merchants. An account of the conversion of the Kerait is given by the 13th century Jacobite historian Gregory Bar Hebraeus. According to Hebraeus, in early 11th century, a Kerait king lost his way while hunting in the high mountains. When he had abandoned all hope, a saint appeared in a vision and said, "If you will believe in Christ, I will lead you lest you perish." He returned home safely. When he met Christian merchants, he remembered the vision and asked them about their faith. At their suggestion, he sent a message to the Metropolitan of Merv for priests and deacons to baptize him and his tribe. As a result of the mission that followed, the king and 20000 of his people were baptized. (R. Grousset, The Empire of the Steppes, New Brunswick, NJ, Rutgers University Press, 1970, p. 191. See also Moffett, A History of Christianity in Asia pp. 400-401.)
The Prester John legend was connected to the Christian rulers of the Kerait ("The history of this race of Christian kings, afterward so celebrated in Europe under the name of Prester John, is properly referable to the two succeeding centuries." (Asahel Grant, op. cit., p. 376)). At one point in the legend, Prester John was explicitly identified with Wang Khan.