In the 7th century, together with Islam, Christianity began to penetrate Central Asia.
The center of its spread was the Metropolis of Merv (modern Turkmenistan), from where Christianity began its victorious march along the Great Silk Road.
It was to the Metropolitan of Merv that the first Kereite Khan Buiruk (order in translation), who united the Kereite union of tribes into a single state, addressed the Merv Metropolitan with a request to convert to Christianity in 1009. At his baptism he took the name Marguz (Mark).
Christianity became a consolidating idea for 400 thousand population of the khanate — very heterogeneous ethnic composition. Among Kereits there were both Mongols — Kereits, Hirkuns, Almads, Dungkhoits, Tumats; and Turko-Uigurs — Jirkins and Sarkaites. All this ethnic mishmash received a single confessional dominance and united into the Kereite community.
This allowed the Kereits to become the dominant ethnos in the Great Steppe at that time. The Kereit Khanate occupied a vast territory from Kerulen, Tola, Selenga to Eastern Mongolia. It bordered the Naiman Khanate in the west, the Merkits in the north, the Tatars in the south, and the Chinese in the east
The state was powerful and rich to the point that the Kereite khans allowed themselves to wear gilded clothes, eat from gilded dishes and live in the same tents with gilding. All this in general was not peculiar to nomads and even Genghis Khan did not allow himself to do it. Most likely it was the result of acceptance of Christianity, where lavish ceremonies and rich decoration are accepted. But to a simple nomad it was incomprehensible and caused bad feelings.
The headquarters of the Kereits was Karakorum, which later became the center of the Mongol Empire.
It was the support of the Kereits that became the key to Genghis Khan’s success. And in the world history they remained as a kingdom of legendary kingdom of Presbyter John, which according to rumors had to come to the aid of Christians and deliver them from attacks of Moslems.
Strangely enough, during the Mongol invasion, the Kereite Christians came to the Middle East to expel the Muslims from the Holy Land — Palestine. But found no support from the Crusaders. Although if the alliance of Mongols and Crusaders would have taken place, then perhaps the whole Middle East would have become Christian.